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Antivirus software consists of [computer program]s that attempt to identify, thwart and eliminate [computer virus]es and other malicious software ([malware]).

Antivirus software typically uses two different techniques to accomplish this:

  • Examining (scanning) [computer file]s to look for known viruses matching definitions in a virus dictionary
  • Identifying suspicious behavior from any computer program which might indicate infection. Such analysis may include data captures, port monitoring and other methods.


Most commercial antivirus software uses both of these approaches, with an emphasis on the virus dictionary approach.

Approaches Dictionary In the virus dictionary approach, when the antivirus software looks at a file, it refers to a dictionary of known viruses that the authors of the antivirus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the antivirus software can take one of the following actions:
  • attempt to repair the file by removing the virus itself from the file
  • quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread)
  • delete the infected file


    • To achieve consistent success in the medium and long term, the virus dictionary approach requires periodic (generally online) downloads of updated virus dictionary entries. As civically minded and technically inclined users identify new viruses "in the wild", they can send their infected files to the authors of antivirus software, who then include information about the new viruses in their dictionaries.

    Dictionary-based antivirus software typically examines files when the computer's [operating system] creates, opens, closes or e-mails them. In this way it can detect a known virus immediately upon receipt. Note too that a System Administrator can typically schedule the antivirus software to examine (scan) all files on the computer's [hard disk] on a regular basis.

    Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing "[oligomorphic code]", "[polymorphic code]" and more recently "[Metamorphic code]" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match the virus's signature in the dictionary.

    Suspicious behavior The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the antivirus software can flag this suspicious behavior, alert a user and ask what to do.

    Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it can also sound a large number of [false positive]s, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the antivirus software obviously gives no benefit to that user. This problem has worsened since 1997, since many more nonmalicious program designs came to modify other .exe files without regard to this false positive issue. Thus, most modern antivirus software uses this technique less and less.

    Other approaches Some antivirus software use other types of [heuristic (computer science)] analysis. For example, it could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use [self-modifying code] or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.

    Yet another detection method involves using a [sandbox (computer security)]. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyzes the sandbox for any changes which might indicate a virus. Because of [performance] issues, this type of detection normally only takes place during on-demand scans. Also this method may fail as viruses can be [Nondeterministic algorithm] and result in different actions or no actions at all done when run - so it will be impossible to detect it from one run.

    Some virus scanners can also warn a user if a file is likely to contain a virus based on the [file type].

    An emerging technique to deal with [malware] in general is [whitelist]ing. Rather than looking for only known bad software, this technique prevents execution of all computer code except that which has been previously identified as trustworthy by the system administrator. By following this default deny approach, the limitations inherent in keeping virus [signatures] up to date are avoided. Additionally, computer applications that are unwanted by the system administrator are prevented from executing since they are not on the [whitelist]. Since modern enterprise organizations have large quantities of trusted applications, the limitations of adopting this technique rest with the system administrators' ability to properly inventory and maintain the [whitelist] of trusted applications. As such, viable implementations of this technique include tools for automating the inventory and whitelist maintenance processes.

    Issues of concern
    • The ongoing writing and spreading of viruses and of panic about them gives the vendors of commercial antivirus software a financial interest in the ongoing existence of viruses. Some theorize that antivirus companies have financial ties to virus writers, to generate their own market, though there is currently no evidence for this. Why there is no global antivirus software conspiracy, by Jonathan Yarden
    • Some antivirus software can considerably reduce performance. Users may disable the antivirus protection to overcome the performance loss, thus increasing the risk of infection. For maximum protection the antivirus software needs to be enabled all the time — often at the cost of slower performance (see also [software bloat]).
    • It is important to note that one should not have more than one antivirus software installed on a single computer at any given time. This can seriously cripple the computer and cause further damage. Microsoft Support
    • It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating [graphics card] drivers. Having antivirus protection running at the same time as installing a major update may prevent the update installing properly or at all.
    • When purchasing antivirus software, the agreement may include a clause that your subscription will be automatically renewed, and your credit card automatically billed at the renewal time without your approval. For example, [McAfee] requires one to unsubscribe at least 60 days before the expiration of the present subscription. In that case, the subscriber may contest the charges with the credit card issuer, but this recourse is likely to fail if in fact the subscriber had authorised such a "continuous payment authority".
    • Some antivirus programmes are actually [spyware] masquerading as antivirus software. It is best to double-check that the antivirus software which is being downloaded is actually a real antivirus program. List of rogue software
    • Some commercial antivirus software programs contain [adware]. For example, the home/small business version of CA Anti-Virus 2008 displays an advert for [CA, Inc.] products whenever the desktop is unlocked after a period of inactivity.


    Antivirus, mobile devices and innovative solutions It would be no surprise when viruses that plague the desktop and laptop world quickly migrate to mobile devices. More and more vendors in this space are offering solutions to combat secure mobile handsets with antivirus solutions. Mobile devices present significant challenges for antivirus software, such as:

    • Processor Constraints
    • Memory Constraints
    • Definitions and new signature updates to these mobile handsets


    SIM, flash based and USB based antivirus products Mobile handsets are now offered with a variety of interfaces and data connection capabilities. Consumers should carefully evaluate security products before deploying on small form factor devices.

    Solutions that are hardware-based, perhaps [USB] devices or [Subscriber_Identity_Module]-based antivirus solutions, might work better in meeting the needs of mobile handset consumers. Technical evaluation and review on how deploying an antivirus solution on cellular mobile handsets should be considered as scanning process might impact other legitimate applications on the handheld.

    SIM-based solutions with antivirus integrated on the small memory footprint might provide a basic solution to combat malware/viruses in protecting [Personal_information_manager] and mobile user data. USB and [Flash memory]-based solutions give the user an advantage to swap and use these products with a range of hardware devices.

    History There are competing claims for the innovator of the first antivirus product. Perhaps the first publicly known neutralization of a wild PC virus was performed by European Bernt Fix (also Bernd) in early 1987. Fix neutralized an infection of the Vienna virus. Kaspersky Lab Virus list IBM anti-virus research timeline First edition of Polish antivirus software mks_vir started in 1987. Program was only available in Polish language version. Autumn 1988 also saw antivirus software Dr. Solomon's Anti-Virus Toolkit released by Briton Alan Solomon. By December 1990 the market had matured to the point of nineteen separate antivirus products being on sale including [Norton AntiVirus] and [McAfee_VirusScan] from [McAfee].

    Peter Tippett made a number of contributions to the budding field of virus detection. He was an emergency room doctor who also ran a computer software company. He had read an article about the Lehigh virus and questioned whether they would have similar characteristics to biological viruses that attack organisms. From an epidemiological viewpoint, he was able to determine how these viruses were affecting systems within the computer (the boot-sector was affected by the Brain virus, the .com files were affected by the Lehigh virus, and both .com and .exe files were affected by the Jerusalem virus). Tippett’s company Certus International Corp. then began to create anti-virus software programs. The company was sold in 1992 to [Symantec] Corp, and Tippett went to work for them, incorporating the software he had developed into Symantec’s product, Norton AntiVirus.

    A very uncommon use of the term "antivirus" is to apply it to benign viruses that spread and combated malicious viruses. This was common on the [Amiga] computer platform.

    See also
    • [Computer insecurity]
    • [List of antivirus software]
    • [List of computer viruses]
    • [List of trojan horses]
    • [List of computer virus hoaxes]
    • [List of Linux computer viruses]
    • [Timeline of notable computer viruses and worms]
    • [Virus hoax]
    • [Virus statistics]


    Notes -->}

    External links
    • AV-Comparatives AV-Test VirusBulletin independent comparatives of several antivirus software
    • Carnegie Mellon's CERT coordination center
    • Virus Information including EICAR/Virus Bulletin
    • Malware definitions


    Information Reference: Wikipedia.org


    Virus checker

    Questions and Answers

    Every time I run an anti virus checker, half way through the computer reboots?

    Q) Any ideas anyone? I have tried using AVG 7.5 and a online checker called Trend Micro HouseCall and they both reboot the computer after about 10 mins of scanning. When the computer boots after the reboot a black screen in DOS comes up saying that I have a hardware or software problem, what would I like windows to do? When i select continue with windows as normal it get to the windows loading screen then it reboots again. If i leave the computer turned off for 5 mins or so the computer seems to boot up again ok. But if i try to run a virus checker again the computer will reboot.

    A) Do a physical check first...make sure your cpu is seated tightly as not to overheat and check the condition of the cpu cooler for dust...clean out if necessary.... I can not tell you how many people thought it was a virus when all it turned out to be was high temps on their cpu shutting the system down and rebooting for no reason...also some turned out to be a faulty power supply... Might want to check into that just in case It could also be a virus or driver issue. Windows 2000 (&xp) have a feature that automatically reboots the computer on certain errors. You can disable this feature as a troubleshooting tool -you will see the error appear. to do this, right click "my computer" and select "properties". click the "Advanced" tab and then the "startup and recovery" button. In here you will see a tick box next to "automatically reboot". Take this tick out and you will likely see the error if caused by the above suggestions. If the computer just acts as if the power has been cut and then comes back on - I would suspect the power supply unit as being faulty.

    Online virus checker?

    Q) My PC is very slow, is there anywhere i can get a really toroguh online virus scan for free?

    A) I use AVG free version and it's very thorough, it takes aaaaaages to complete and scans every nook and cranny of your pc! You could also try deleting your cookies, and running the de-frag thingy, that can help speed things up a bit. :-)

    what is the best FREE trojan / antispyware and virus checker and deleter.?

    Q) My computer has gone mad and none of the programs tried has fixed it, and one I tried was a virus itself - a fake antivirus!

    A) Your question tells me you had an adware program that informed you of an infection and to buy their product to remove it. This is a common problem. The way this usually happens is you get a Trojan on your computer. A Trojan is not a Virus and/or Spyware. It is a seperate form of malware. It looks innocent and therefore, may be allowed on your computer. It then downloads the adware or other forms of malware. Once the Trojan starts doing its job your AV/AS programs will recognize it and alert you. Your only choice is to remove it and the associated malware. These types of infections can require you to try a few different programs to remove them. This site is very good at removing many of the Trojans that do this. This site has become very well respected for its ability to clean your computer of Trojans and does it free. http://www.internetinspiration.co.uk/roguefix.htm Vundo Trojans are known for installing this form a adware also. It is is usually associated with the Winfixer series of rogue Antivirus and Antispyware program but, can have other also. The procedure should fix Vundo-based Winfixer(WinAntiSpyware, WinAntiVirus, Blackworm, Amaena)problems. Please download Atribune's VundoFix.exe (version 4.2.71 [as of 21 April '06], or later), from http://www.atribune.org/ccount/click.php?id=4 and save it to your desktop. Double-click VundoFix.exe to run it. Put a check next to Run VundoFix as a task. You will receive a message saying vundofix will close and re-open in a minute or less. Click OK Note: If VundoFix does not reopen after a minute (or two), then you should skip-over the "Run as a task" step, and continue-on to the following steps to SCAN and REMOVE. When VundoFix re-opens, click the Scan for Vundo button. Once it's done scanning, click the Remove Vundo button. You will receive a prompt asking if you want to remove the files, click YES Once you click yes, your desktop will go blank as it starts removing Vundo. When completed, it will prompt that it will shutdown your computer, click OK. Turn your computer back on.

    Can anyone recommend an up to date PORTABLE anti-virus checker?

    Q) I need some anti-virus software I can just copy to CD and run on a PC regardless of it's condition. Can anyone RECOMMEND anything that portable, or suggest a way I can make some AV software portable???

    A) McAfee Stinger http://vil.nai.com/vil/stinger/ Avast Virus cleaner http://www.avast.com/eng/avast-virus-cleaner.html

    Can anyone recommend an up to date PORTABLE anti-virus checker?

    Q) I need some anti-virus software I can just copy to CD and run on a PC regardless of it's condition. Can anyone RECOMMEND anything that portable, or suggest a way I can make some AV software portable???

    A) McAfee Stinger http://vil.nai.com/vil/stinger/ Avast Virus cleaner http://www.avast.com/eng/avast-virus-cleaner.html

    I need a free virus checker to replace Nortons which one is easiest to use?

    A) I use AVG free edition, its quite good, easy and light.

    anyone recommend a free virus checker that will not slow my computer down too much?

    A) AVG Anti-Virus

    How well do you rate virus checker NOD32?

    A) 10/10 it does everything every other anti virus does but better....it updates multiple times a day, and its just the right anti virus to go with, i use it and ive never had a virus ever since...

    Slow Computer - is it my virus checker or my broadband?

    Q) My computer is running really slowly and i don't know if it is my Windows OneCare or if it is my Broadband which is BT Yahoo - can anyone help?

    A) try using ccleaner it speeded mine up by loads

    how do i removing norton virus checker?

    A) as in remove the program northeran anitviris??? select START, CONTROL PANNEL, ADD/REMOVE PROGRAMS, click the program and follow the instructions

    Bootable CD Virus Checker?

    Q) HI I need a bootable CD Virus Checker (As in 1 that is up-datable) Preferably a "Free / Shareware One" I have AVG Free 7.5 But need to create a bootable CD that wll start a PC and do the check Any Ideas Guys ? TIA

    A) Well.....almost all Brand Antivir software have now a bootable version . Norton Antivirus , Bit Defender and there are a lot more . BUT....in ur case u can use as well an ON-line scaning antivir . Is ABSOLUTELY free to use . Just search in a search engine like yahoo or google for """ free online antivir """ If you wanna know more..... Feel free to contact me on yahoo messenger for more details.

    what happened to the virus checker on the toolbar?

    Q) ...we just downloaded the "new and improved" toolbar, but the virus checker is gone

    A) It is no more! Yahoo decided to give us some funky trial virus software! It sucks!

    Which is the BEST virus checker?

    Q) Which virus checker prog is the best one? Least active memory use, least system drain, and best protection. Right now Im running avast, which seems to work just ok. Any suggestions? I have Avast, which works fairly well, and is also farly easy to use. I've always had problems with Symantec, so no Norton, although it has also worked well in the past. Mainly, Im looking for something that has good coverage for your average use, plus easy interface...

    A) There are many decent anti-virus programs available. Norton's is one of the best, as it's both thorough and economical and doesn't use up a lot of memory; however, the problem where memory is concerned is that the best programs require the most memory. I was using Nortons awhile back and still got a virus, so I spent about $300.00 on a very extensive program called Tech on Demand. Unfortunately, it requires a lot of memory to run because of the many functions of the program. The amount of memory is pretty much proportional to the amount of protection you're looking for.

    i have a new Norton virus checker. How do I un-block the Pop-up blocker?

    Q) After I installed a new Norton anti virus program i cannot view broker account statements and am asked to "un-block" the pop up blocker. I am new to this and have never experienced this. What do I do - where do I find the pop up blocker to un-block it?

    A) 1.go to tool 2.scroll down to pop up blocker 3.then go to turn off pop up blocker

    My virus checker came up and said I had a pup on my pc what is a pup please?

    A) PUP is a generic term that stands for "Potentially Unwanted Program" Your virus checker is telling you that there is a file on your computer that you may want to eliminate. Usually these refer to adware or spyware programs planted by something that you've downloaded. You can safely eliminate it using your virus checker.

    free anti virus checker?

    Q) Does anyone know of one besides AGV?

    A) It's best to scan with several virus/spyware programs. There's several online ones free that you can use. I like panda and bitdefender best because they've found the most viruses. Bitdefender is slower but supposedly high rated. Norton causes crashes on startup sometimes. AVG is a lemming that tells you where the virus is and doesnt really remove it. Adaware is best for removing spyware. http://www.jimmyr.com/security.php

    What's a good " virus" checker??

    A) I am using Mcafee, which is rated in the top 10, but I have read where there is others as good or even better but you can find out the best by going to Cnet.com, I'll stick with mines for now, it can be mind-boggling at times.Sometimes those free stuff have a lot of bundles or takes a lot of resources from your PC, but I have used and would recommend AVG(a free anti-virus software) if that is what you really want, I am paying for my service which offer me "live" support in case of any trouble that's a help, plus they took over the siteadvisor format which is a free service of this company, this prevents me from going to site that a loaded with spyware and adware which can be more damaging than a virus a lot of times.

    Why won't my virus checker work?

    A) Here you have some good and free antivirus programs: http://askcomputerexpert.ws43.com/download/security.htm Get one!

    what security does Yahoo provide--virus checker, firewall, spam??

    Q) Anything else and is it automatically on or do I have to do something to activate?

    A) ALTHOUGH OFFERED AS A FREE SERVICE, THE COMMON PACKAGE SECURITY DEALS ARE USUALLY NORTON, OR OTHER MAJOR BRAND WITH THE HOPES OF GETTING YOU TO UPGRADE TO THEIR BIGGER PAY AS YOU GO ITEMS. DON'T FALL FOR IT. THESE ARE TOP HEAVY BLOATWARE THAT DEEPLY 'HOOK' THEMSELVES INTO THE KERNEL (THE HEART OF XP) AND OFTEN SLOW DOWN A SYATEM. GET SOME FREEWARE ('SNAPFILES.COM/FREEWARE ' IS A TERRIFIC SITE AND RATE VARIOUS OFFERINGS.) TRY 'AVG' OR 'AVAST' FOR ANTI VIRUS; 'AD AWARE' AND 'SPYBOT S&D' ARE VERY NICE; 'SPYWAREGUARD' AND 'SYPWARE BLASTER' ARE WORKING WELL FOR ME. SECURITY AND PRIVACY ARE VERY CRITICAL: CHECK OUT THE 'SECURITY NOW' RSS FEED FROM LEO LAPORT AND STEVE GIBSON: TIME WELL SPENT.

    i'm thinking of installing a different anti-virus checker?

    Q) and i was wondering what happens to the viruses that are held in quarentine during the change over. do i have to delete them, is it safe to do so, or do they go into quarentine on the new programme?

    A) When you uninstall any anti virus , the quarenteen ones will be let free in your hard drive. Anyway the so called quarenteened ones are suspect only or cannot be deleted due to their firm attachement to a file or shared files. SO If possible if a file is affected , just delete them and re-install new ones again.

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