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?
If it does not update (yet), please, post the last 200-250 lines of avast log: C:\Program Files\Alwil Software\Avast4\DATA\log\Setup.log