Hello and Welcome to Windows7forums..
I would recommend either doing a system restore to the point before you tried to downgrade.. OR if that's not a possibility you may have to reinstall Windows 7 (via clean install)..
I have a funny feeling your going to run into more problems as a result of making modifications to the registry.. They probably can be fixed but messing with the registry is never fun or advised if you don't know exactly what your doing..
Hope this helps out..
Cheers for now..