Logging Off Windows XP

Just a quick question about XP, I recently installed, and activated XP on my home system. Ran the O/S for less than a week when I ran into a logon problem. As my profile is loading my o/s loggs me out. I’ve tried every possible way of logging in with out any success. I don’t want to re-install and activate again seeing I am limited to the amount of activations. The guest account has also been disabled. ?? what to do ? Im tempted to revert to 2000 pro.