If you have the Professional Edition, you can do this easily by editing the resident task in the Enhanced User Interface.
For the Home Edition, unfortunately, there is currently no easy way to do it. What you can do is rename the file ahResOut.dll in the avast4 folder (to prevent it from loading), but be prepared that it will be eventually put back by every program update.
Also, you'll probably first need to kill all avast components as otherwise this file will be locked.
Hope this helps,
Vlk