Which is your avast version: Professional, Home or Trial.
Please, post the last 100-150 lines of avast log: C:\Program Files\Alwil Software\Avast4\DATA\log\Setup.log
Which is your Windows? 98, Me, XP?
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). You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?