what do u mean by a reg clean?
Use a Registry Cleaner application.
You can Google for freeware ones. You can click on 'Links' in my signature.
But, you can try to install avast just uninstalling AntiVir first and booting before installing avast. Try to see if it works
![Cool 8)](https://forum.avast.com/Smileys/default/cool.gif)
Let the Registry Cleaner for a second tentative
![Smiley :)](https://forum.avast.com/Smileys/default/smiley.gif)