I’d recommend running a clean install:

1.Download your version of avast and the avastclear tool - save them to the desktop:

Avast Free Antivirus
Avast Pro Antivirus
Avast Internet Security
Avast Premier

Download Avast Clear Tool

  1. Uninstall Avast from Control Panel (if possible) and reboot.
  2. Uninstall in safe mode using Avastclear. [Running Avastclear is normal mode will give a prompt to go into Safe Mode]
  3. Install the version you downloaded.
  4. Reboot.

Paid: If you have a paid license, insert the license key/activation code after the reboot at stage 6.