Hi we will in this instance go for a full clean install
Download Rejzor’s 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
[]Unzip Rejzor’s tool to the desktop
[]Run the tool and accept the usage
[]Once complete reboot your system
[]Reinstall Avast