A Trojan is a type of threat named after the Trojan Horse of Greek mythology. Like the Greek Trojan Horse, a Trojan network threat has malicious intent, hidden within its code. While a Trojan may appear innocent, executing a Trojan can cause unwanted system problems in operation, data loss, and loss of privacy.
For example, a Trojan called "happy birthday" might play a
song and display an animated dance on your screen, while at the same time
opening a port in the background and dropping files that lets malicious
hackers take control of the computer for whatever scheme or exploit they
may have in mind. One common scheme is to hijack the computer for distributing
spam. Another is to collect keystrokes and send them
Trojans are not viruses. Unlike viruses, they do not infect files, and they do not replicate. Because a Trojan does not infect a file, there is nothing to clean, though the scan engine may report the file as "uncleanable" and delete it, quarantine it, or take whatever action you specify.
With Trojans, however, deleting or quarantining is often not enough to rid your system of the Trojan’s effects. You must also remove any programs that may have been copied to the machine, close ports, and remove registry entries.