After you have done what Asyn mentioned. Go to this website http://rejzor.wordpress.com/avast-cleanup-tool/ and scroll down a bit and you will see a Zip file called Avast Cleanup Tool. Click on that and download it. Run that tool in normal mode and restart manually. Once you have done that reinstall a fresh new copy of Avast and follow instructions. After that Avast will do a startup scan automatically. After it finishes the startup scan. Restart your computer again manually. After restart register your copy of Avast :).