This question could be coming from a person that is using a Microsoft known pirated version of Windows XP Professional Corporate Edition. Since I have no way of verifying any claims made to me via E-Mail, I ignore all "questionable" requests for such type of information.
If you purchased a Legitimate copy of Windows, this should not occur and there would be no real reason to ever have to change your product ID or your serial number. Contact Microsoft Technical Support for assistance if you do have a valid reason to do such a task. The contact information is located in your manual that came with your particular version.
If you are using a pirated copy of Windows XP, please do not ask me for help. I will not answer any questions with regards to pirated software.