How to delete ALL of old user

I cloned my workmate's XP SP1 hard drive since my system won't boot to a CD. That worked fine and it booted fully, since he had never entered a password for his user. I updated to SP2 fine. Then I created a new user for me, also with no password, and deleted his old user. Even after I restarted, it flashed a "can't login" message with a greyed out dialog box behind with his user name. I clicked "Okay" and it went right past the login and all was fine. I got curious about why it has now flashed the login error for a deleted user, so I searched files for his name and that turned up dozens of things. It seems Windows doesn't really know how to delete a user. I thought I would take matters into my own hands (stop laughing) and trash the folder for his name that was in (Documents and Settings". I got lots of warnings about deleting system files, but that seemed impossible, since these files aren't in the system area (WINNT). Well, you guessed it, got a hang at next startup after the "can't login" message was dismissed. Boots to Safe Mode fine, so I went to his computer and copied that folder to a mini drive, copied it to my Docs $ Settings folder, but it made no difference. Must be related to the SP2 update? I suppose this is either a registry thing or some other database. What should I do, short of re-clone?