Anti-virus software can defend you against viruses, Trojans, worms and – depending on the product – spyware and other types of malware.

Anti-virus software uses a scanner to identify programs that are, or may be, malicious. Scanners can detect:

• Known viruses – The scanner compares files on your computer against a library of “identities” for known viruses. If it finds a match, it issues an alert and blocks access to the file. 
• Previously unknown viruses – The scanner analyzes the likely behavior of a program. If it has all the characteristics of a virus, access is blocked, even though the file does not match known viruses. 
• Suspicious files – The scanner analyzes the likely behavior of a program. If that behavior is of a kind usually considered undesirable, the scanner warns that it may be a virus.

Detection of known viruses depends on frequent updating with the latest virus identities. There are on-access and on-demand scanners. Most anti-virus packages offer both. On-access scanners stay active on your computer whenever you are using it. They automatically check files as you try to open or run them, and can prevent you from accessing infected files. On-demand scanners let you start or schedule a scan of specific files or drives.