Definition: http://en.wikipedia.org/wiki/Trojan_horse_(computing)
This is not a virus you want on your computer. It can be written to do almost anything. It can even help spread other viruses.
PREVENTION> You must have the best Anti-Virus program you can find. I am not in the business of promoting one or the other. However I do not recommend the free ones. See: http://www.download.com/Antivirus-Firewall-Spyware/?tag=dir
Viruses, spy-ware etc are way too aggressive to play games with. Nobody wants to spend money on prevention. Yet it would cost you a lot of money to have a professional to remove the infection. In some cases these “enemies” WILL destroy your hard drive.