I'd suggest doing a clean install.
1.Download your version of avast!, the avastclear tool and save them to the desktop:
Avast Free AntivirusAvast Pro AntivirusAvast Internet SecurityAvast PremierDownload
Avast Clear Tool2. Uninstall Avast in Control panel [If you don't have Avast in control Panel go to #3]
3. 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.
5. Install the version you downloaded.
6. Reboot.
Paid: If you have a paid license, insert the license key/activation code after the reboot at stage 6.