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, loss data, 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 he or she may have in mind. One common scheme is to hijack the computer for distributing spam. Another is to collect keystrokes and send them, along with all the data they contain, to the malicious hacker.

Trojans are not viruses/malware. Unlike viruses/malware, 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, simply deleting or quarantining is often not enough to rid your system of the Trojan’s effects. You must also clean up after it; that is, remove any programs that may have been copied to the machine, close ports, and remove registry entries.