I’d suggest doing a clean install.

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

Avast Free Antivirus
Avast Pro Antivirus
Avast Internet Security
Avast Premier

Download Avast Clear Tool

  1. Uninstall Avast in Control panel [If you don’t have Avast in control Panel go to #3]
  2. Uninstall in safe mode using Avastclear. [Running Avastclear is normal mode will give a prompt to go into Safe Mode]
    Check : Once uninstalled check in device manager>view>show hidden devices if there is anything related to avast with a yellow triangle… if so, uninstall it and reboot.
  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.