Updating Windows Serial - Need help ASAP

Friend of mine got in trouble for using a "copy" of Windows XP Pro (auto-update and uh oh, first warning), I persuaded her to buy a genuine one but instead of re-installing the whole thing how do we upgrade to the new legit serial?

I know it's possible I've just never done it help plz...

EDIT - Nevermind, found this: http://support.microsoft.com/kb/328874 (Can a mod lock or delete plz?)

What about machines with XP SP2 installed? Can the same method be used?

ICE