There could be various reasons why your computer still can be infected but lets look at the most obvious.

You firewall obviously let it in at one point or maybe it was there before the firewall. Either way it is on your machine. Have you tried to google the name and see how to delete it. Because sometime AV proggies will not totally delete the file from your reg and then it will come back on a reboot or on access a certain something. I would google the trojan name and see what you get.

You should look into how you Firewall is configured you may need to reconfigure if it is letting this trogan in. Or it is on your trusted list I would double check the sites you allow (you gotta be careful with porn sites....I know I know its fun but be careful).

By the way what is the name of the Trojan

- MilitantEidolon