Antivirus software is the programs that try to identify, unarm and eliminate malicious software, including different types of viruses, trojans, malware etc. Antivirus applications usually do it in two ways:

- they virus scan files to find the viruses matching the definitions of their virus dictionaries;
- they track the suspicious activity in the process tree of your system, trying to identify the files causing this activity and remove them;

Anti virus software is usually equipped with the built-in virus scanner to perform the profound virus scan. If such application finds a piece of any file's code matching an entry in the virus dictionary, it usually tries either to remove the malicious code from the file (repair it), quarantine the file so no programs can access it (thus preventing the virus from spreading) or deletes the infected file in case it cannot be saved.

Sometimes virus scan is performed the following way: the virus scanner analyzes the code of each new executable and if it finds out the executable is trying to change its code or starts searching for other executables to modify, the application tags this executable as potentially dangerous. Then the more thorough virus scan is initiated to make sure there is no mistake.

Virus scan practices surely contribute to the global war against viruses and other malicious software. However, many experts agree that most of the common issues associated with viruses could be easily solved without virus scan and fight activities. The problem is that modern e-mail applications, such as Microsoft Outlook Express or The Bat! still have numerous bugs which allow the e-mail viruses (which are undoubtedly the most destructive and rapidly spread malicious software) to install themselves without asking for user's consent. However, much depends on the users, too. It is advised to educate your personnel so that they never open and execute unknown files and applications.

Sometimes viruses and other malicious applications are encrypted which makes them almost impossible to track with the help of the ordinary virus scan. In order to detect such a virus, The virus scanner first needs to decrypt the harmful file, which takes noticeable system resources. That is why many anti virus applications today do not possess the decrypting function, so you may never be sure your system is clean. Maybe it's because of your virus scanner is blind to see that smart virus. Luckily enough, there are not much of such encrypted viruses available today, because they need much more system resources and brainstorming to be designed, thus being inaccessible for those lacking these resources (or brain, whatever). 

Virus scanners may slow down the performance of your computer, but you should always remember that the process of virus scan is vitally important for the health of your system. Otherwise, you risk to find yourself sitting at the shipwreck, unable to use your pc, because you have just turned off your protection for an hour or two. Virus scan is important. That is obvious.