Practically everyone with a computer has heard of computer viruses, but not everyone knows exactly what they are or how viruses can affect their computer. A virus is a small, self-replicating program that invades your system through an e-mail attachment, download, or Web site. People create viruses for various reasons, ranging from mild pranks to malicious destruction.

As a result, the effects of a virus can vary from moderately annoying to absolutely crippling. The Wazzu virus, for example, attaches itself to Microsoft Word documents and inserts the nonsense word "?wazzu?" in the document and rearranges random strings of words. A virus such as this one is definitely annoying, but not necessarily catastrophic.

Some viruses can be much more serious, however, threatening to destroy data or even erase your entire hard drive. One such virus spread by e-mail, Worm.ExploreZip, takes control of the infected computer's e-mail system and automatically responds to all incoming mail messages. It also sends destructive e-mail attachments that can erase certain types of files.