Networking World

Named after the famous Wooden Horse used by the Greeks toconquer the city of Troy, these programs basically invade your PC disguised as a piece of desirable software. Often hidden inside the installation routines of pirate software downloaded from the internet they install themselves at the same time as the actual application and modify your system settings to ensure they are automatically run every time your PC is started up.
Trojan horses themselves seldom damage the system they infect, preferring to silently allow attackers to access your machine over the internet and do almost anything from stealing your files through to destroying your hard disk. Because they normally rely on an internet connection, PC's with cable modems or other always-on connections are especially vulnerable if infected with a trojan horse.