In order to convert a trial version of Windows XP Professional
to a fully licensed version, you'll need to purchase a "Full Version"
of Windows XP Professional and proceed with a "Repair Install".
(You cannot simply change the Product Key).
You can't just change the PK. You will have to purchase a full retail
version of XP and then perform a 'Repair' install, using the new PK when
requested.
Please see the following article by Michael Stevens regarding a 'Repair'
install: