Computer Viruses are malicious programs designed to self-replicate and spread to other systems. Their function can range anywhere from displaying a message on a certain date, to permitting others to control your computer, to erasing files or formatting your hard drive. Trojan Horses are similar, except that a trojan horse is defined as any malicious program disguised as a harmless one. (Which means that all Computer Viruses are also trojan horses, but not all trojan horses are viruses) The best protection against either viruses or trojan horses is to be very careful not to run any untrusted files, and to run an up to date virus scanner regularly.