Most viruses do not do much, or any harm. Many exist just for the sake of proving that the author was capable of creating a virus that can spread from machine to machine. Some non-damaging viruses may simply be tests of the virus code’s ability to spread so the author can work out any bugs before adding the destructive payload.
Even a virus that does no inherent damage to an infected machine can have a huge negative impact on the Internet at large. In late January of 2003 a worm hit the Internet called SQL Slammer. It took advantage of a known vulnerability in Microsoft SQL Server and spread around the world in less than an hour.
It did no damage to the infected machine per se, but the speed and volume of connections that an infected machine initiated grew exponentially as more machines became infected and caused so much congestion of traffic that it all but shut down the Internet for a few hours.
To guard and protect your computer from known virus, worm and other malicious code threats you need to have antivirus software installed and actively running. You also must keep it updated. New viruses are discovered almost daily. Running antivirus software that hasn’t been updated in a month or more is equivalent to not running antivirus software at all.
The antivirus vendors analyze new malicious code threats as they are discovered. They look for pertinent information that makes the threat unique- size of the file, specific text in the file, message body or subject line, specific ways the file works, etc.- and create a signature or footprint that will identify this threat. These signatures are included in the update files put out by the antivirus vendors. Most vendors update their virus definitions at least weekly.
Identifying the signature of the threat and including it in the updated virus definitions is great for catching known threats, but how do you stop a virus or worm that hasn’t yet been detected and catalogued by the antivirus software vendors? To do this, most vendors use heuristic analysis. Heuristics uses past experience to make educated guesses about the present. Using rules and decisions based on analysis of past network or email traffic, heuristic scanning in antivirus software can self-learn and use artifical intelligence to attempt to block viruses or worms that are not yet known about and for which the antivirus software does not yet have a filter to detect or block.
There are many products available on the market to help you protect your computer. McAfee VirusScan from Network Associates and Norton Antivirus from Symantec are two of the most recognized names in antivirus software. There are plenty of other options though such as Sophos or F-Secure as well as free options for those who want to protect their computers on a tight or non-existent budget.