I tried this feature. The screen became very confuse, trying to be set to 640 x 480 pixels...
I could solve this working with PowerControl 1.02 (Freeware,
http://smsoft.chat.ru/en).
The final configuration is:
Screen saver: PowerControl
Click, Configurations (it´ll be available)
Chose the screen saver avast! antivirus (into PowerControl).
You´ll get access to the avast! antivirus settings.
After this, you can change all settings: Windows, PowerControl and avast! antivirus.
By the way, this feature is not so stable than the same in another antivirus.