Uninstall Avast
Download the Avast Uninstall Utility to your Deskto http://www.avast.com/en-gb/uninstall-utility
For Windows XP and Vista get the Avast Removal tool from here https://install.avcdn.avast.com/iavs9x-xp/avastclear.exe
Uninstall Avast via control panel and follow the directions.
Run the Uninstall Utility and accept the reboot to safe mode
Once complete reboot your system.
A video to help you use the Avast Uninstall Utility is available at: https://youtu.be/KD8LO8OxmvM