There is no need to format the HDD.
This has nothing to do with the Avast installation but rather a little known bug which happens sometimes with avastclear.
What the OP is saying is, when he/she boots the PC up, there is a dialog to choose the operating system or Avastclear. They get the choice of Windows 10 or Avastclear which shouldn't happen since he's already ran Avastclear. This bug happens sometimes.
In order to fix this:
You need to open up "System Configuration" > type "msconfig" from Run or Cortana option. Then go the "Boot" tab.
Avast Clear Uninstall Utility or similar will be listed, click it and select the delete option.
Then click "Apply" and "Ok" and the issue will be resolved.
You may be requested to reboot so ensure you do if required.