I have been useing Tauscan, a product of Agnitum. It detects and removes Trojans. Simular to a Anti-Virus it uses a database of signatures which need to be updated regulary. Agnitum also offers a Firewall called Oupost, I dont use thier firewall but I do recomend Tauscan for detecting and removing trojans. It also has a memory resident portion of the scaner that constantly monitors in the background for Trojan hourses. I recomend it. In my opinion it is worth buying.

http://www.agnitum.com/download/