Similar to their biological brothers, computer viruses are designed to propagate, traveling from computer to computer to perform some mischief. That mischief may be as innocent as displaying a message that reads, "This Computer is Stoned," or as fatal as wiping out every file on a hard drive. There are more than 48,000 known viruses, and 1,000 or so are active.

ny program installed on your computer that does bad things. Some will send personal information (passwords, etc.) back to the originator; some install viruses, trojan horse programs, etc.; some change your homepage to theirs. There are many of these.