Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have that might have an effect, anti-spyware, etc. SpySweeper, Spyware Doctor, WinPatrol, ProcessGuard, etc. ?
What is your CPU usage in Task Manager, what is using most ?
Is the avast icon rotating in the system tray ?