Hello billdac,
If you have a Genuine copy of XP and you want to use that on a computer that has a non-genuine copy of XP on it, you would need to use the Product Key update tool (
http://www.microsoft.com/genuine/selfhelp/XPPkuinst.aspx) to replace the bad Product Key with the Genuine one. Note: if the Product Update tool does not work, you will need to reinstall XP using the Genuine XP software and product key.
Thank you,
Darin MS