This is a topic from 2004…
Which avast version are you using?

Which is your Windows? 98, Me, XP, Vista, 7?
Do you use any other antivirus in your computer?

Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair).
If this does not help, can you uninstall / boot / install / boot again?