Which is your Windows? 98, Me, XP, Vista?
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?
Which was your old antivirus? How did you uninstall it?
Which other security program are you running?
Are there ash*.exe programs running (ctrl+alt+del to check them)?