Help with service pack two problem

Just installed SP2 on a pc running xp home. All went well until rebooted
when it says you must activate to log on - clicked ok to activate and it
loads the windows activation screen then says that it is already activate
and logs back off, tried logging on again and same thing happens.

Tried uninstalling sp2 in safe mode and says it can find a file and won't
uninstall. Tried system restore but won't have that either.

Any help would be much appriciated before i lose more hair over this