You need to perform a clean install:
Note: It's important you used the stub online installer from the one I linked.. NOT the offline one.
Note: You need to be ONLINE during this install (online installer works in all cases whereas offline sometimes doesn't)
1.Download your version of avast! and the avastclear tool:
Avast Free AntivirusAvast Pro AntivirusAvast Internet SecurityAvast PremierDownload
Avast Clear Tool2. Uninstall Avast by 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.
Free: If you have the Free version, register the product for free for a year from the avast! UI.
Paid: If you have a paid license, insert the license key/activation code after the reboot at stage 6.
If you have any questions, please don't hesitate to ask. It's better to ask than make a mistake and start over again with the process.