A Trojan Horse is a kind of computer program that is used to break into a computer by masquerading as something else. It is named after the Trojan Horse, a trick used by the ancient Greeks to invade Troy, as the Trojans believed the horse was a harmless gift. These programs work by lurking within other programs, but they do not invade other files or spread like computer viruses do.
Trojan Horses are often spread by social engineering—the manipulation of the computer user into thinking the files are safe. For example, a Trojan Horse can be spread by sending a user a form that looks harmless but isn't. Some examples of Trojan Horses are backdoors, which send information to a user who is remote from the computer. The remote user then has total control over the computer and can send, receive, change, or delete files. The infected computer can also be used as a "botnet" to send spam or as a way to conduct attacks or carry out illegal activities with other computers as the targets. One particularly noxious kind of Trojan Horse promises to destroy viruses on your computer but instead installs them.
To recover from a Trojan Horse, you should disconnect your computer from the Internet, back up files, and then scan your system with an antivirus program. Once you've found and removed the corrupted files, you should reinstall your operating system. To prevent being affected by Trojan Horses, do not open attachments in e-mails that seem suspicious or that you did not ask for. In addition, install and update antivirus software on your computer, and install an Internet firewall.