I suggest an installation from scratch:
1. Download the latest version of avast! Uninstall Utility
and save it.
2. Download the latest avast!
version and save it.
3. Uninstall avast from Control Panel (if possible). If, for any reason, you can't run it, try booting in Safe Mode
and doing it from there. Anyway, boot after that.
4. Run the avast! Uninstall Utility saved on 1. If, for any reason, you can't run it, try booting in Safe Mode
and doing it from there. Anyway, boot after you've run it.
5. Install avast! using the setup saved on 2. Boot.
6. Register your free copy or add the license key for Pro. Or even upgrade
your key from old versions. You can register it directly from avast interface > Maintenance.
7. Check and post the results. If, for any reason, you did not solve, try doing the step 3 in Safe Mode anyway.