Similarly, what is known as a Trojan horse is a destructive program disguised as a simple application or useful program. The program contains additional hidden code which allows the unauthorized collection, exploitation, falsification, or destruction of data. Though lumped in with viruses, it doesn't propagate itself like a virus does.

One of the worst kinds of Trojans is a program that claims to rid your computer of viruses but instead puts them on your computer. Another common type will open a security hole for a hacker to get into your system and do all kinds of nasty stuff

You can prevent getting a Trojan Horse by having up-to-date anti-virus software, not opening unsolicited attachments, and scanning freeware and shareware you download before you unzip.