Changing XP Product Key

I’ve created a bootable WinXP CD slipstreamed with SP2 and hotfixes from a Dell XPS400.
I’ve formatted and installed WinXP using the CD on a Dell Dim4400.
How do I change the XP product key to the licensed Dim4400 one?
I’m afraid to try activating/windows update on the Dim4400 in fear it will screw up my original XPS machine license.