A Trojan, short for trojan horse, is a malicious program that masquerades as being a legitimate program. Often users are tricking into insalling the Trojan horse thinking it is a file from a genuine source. One of the most insidious types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.
Trojans differ from viruses in that they do not replicate themselves, nor do they reproduce by infecting other files. However they can be just as destructive. Trojans are known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information such as passwords for bank account information to be compromised without your knowledge. I recommend using a Trojan Scanner along side your Anti-Virus application.
Free Trojan Scanner
A-Squared
Online Trojan Scanner
WindowSecurity.com Online Trojan Scan