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 it fails, I suggest an installation from the scratch:
1. Uninstall avast from Control Panel first.
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.
5. Install again the latest
avast! version.
6. Boot.
7. Check and post the results.