If its a genuine CD key, then you have to contact Microsoft's technical support and tell them the problem and send them your key/COA label photo along with your registration details (the one that you used to register Windows XP with) However, if you dont have a COA label on your PC or in your manual, then its probably a pirated key and since SP2 rigorously checks for known bad keys, it wont allow you to proceed with the installation without a legal CD key.