No, I am not connected to Avast, just another person who has their Internet Security installed on both of my computers. Perhaps I can help you with your questions, if not then Please contact their Support.

Can you use Last Known Configuration so at least you can get back on your computer and then remove the registry keys, etc that was left over on your computer? If you can, then use either CCleaner or ReveUninstaller to remove the rest of the left overs of Avast off your computer.