Are you using Free or Pro! If you are using Free. Uninstall it via add/remove, restart it, and use this tool http://www.avast.com/uninstall-utility and follow the instructions. About need to re-register after re installing Avast the answer is yes you need to re register avast using the same data that you used before. I don’t know about Pro if you need to re register because i don’t that know and I am using Free version.