, I don't think you are going to like this, but...
This is what Microsoft says, http://support.microsoft.com/kb/307545
Now, that is not probably what I would do. I think I'd try a repair install of XP (35 minutes), rather than try all those different commands where a single keystroke in the wrong place...could have negative consequences.
As for not having an XP CD...I would (if I didn't already have several copies) borrow one or copy one and use it with your bona fide activation data for that system.
FWIW: My few attempts to follow such commands to repair the registry...all proved futile and I wound up doing repair or clean installs.
Hopefully, others here can provide additional alternatives.