Hi spookytone,
What was the name and location of the infected file reported by avast?
In addition to Ad-Aware, I suggest you try Spybot Search & Destroy (another anti-Spyware program) and Ewido and/or a-Squared anti-Trojan programs (Ewido requires Win2000/XP).
Make sure you update before scanning.
Run these in safe mode if possible (Tap F8 while rebooting).