I would recommend a clean install
Download Avast Uninstall Utility to your Desktop.
Download the correct version of Avast
Avast Free
Avast Pro
Avast Internet Security
Avast Premier
Disconnect from the net
Uninstall Avast via control panel
[]Run the uninstall tool and accept the reboot to safe mode
[]Once complete reboot your system
[*]Reinstall Avast