A computer virus is a malicious computer program that infects the computer memory or hard disk. Some are comical and will simply change the display on your monitor while most are designed to do damage to the data on the computer. Boot sector viruses are especially damaging since they rewrite the information needed to start up and access your computer’s hard disk. Trojan horses are destructive programs disguised as something harmless like a game. Worms are programs written to replicate themselves until the computer’s memory or hard drive is completely used up at which time the computer crashes and becomes inoperable.