You can be 100% sure of its demise by zero filling and reformatting your hard drive. Who knows how far that trojan has gotten. Its a time consuming task and should be used as a last resort (considering you have the software to reinstall everything). Follow the advise of dave m, I have avast and have had no problems at all. Above all Avast is 100% free. Yes even after the 60 day trial.
You may also be well advised to use two of the freebie Anti-Spyware programmes. I use Adaware and Spybot. They both tend to spot different things and they tend to pick up trojans better than the antivirus programmes. The free AVG antispyware program comes well recommended too. All can be found by googling.
hi roy sometimes these things hide in system restore so do keep coming back after you think you removed it. have a look at this page it explains what to do in this situation. hope it helps.