Apparently Microsoft doesn't object to you changing CD-keys... they even tell you how to do it (if you've got the VLK at any rate)...
http://support.microsoft.com/default.aspx?scid=kb;en-us;328874&Product=winxp
Careful you don't do this with an invalid key... you could break Windows this way and then *have* to reinstall anyway...
http://support.microsoft.com/default.aspx?scid=kb;en-us;328874&Product=winxp
Careful you don't do this with an invalid key... you could break Windows this way and then *have* to reinstall anyway...