1. Uninstall avast from Control Panel first (if possible).
2. Boot.
3. Download the latest version of
Avast Uninstall and use it for complete uninstallation. If, for any reason, you can't run it, try booting in
Safe Mode and doing it from there.
4. Boot.
Did you use any other antivirus before avast? How did you uninstall it?
Did you try repairing your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair).