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).
Please, upload (attach) the avast log:
C:\ProgramData\AVAST Software\Avast\log\Setup.log
or C:\Program Files\Alwil Software\Avast5\Setup\setup.log
If the file is too big for the forum, post the last 400-500 lines of it.