Does this thread help you? http://forum.avast.com/index.php?topic=28267.0

Can you try an installation from the scratch?

  1. Uninstall avast from Control Panel first.
  2. Boot.
  3. Use Avast Uninstall for complete uninstallation.
  4. Boot.
  5. Install again the last version.
  6. Boot.
  7. Check and post the results.