You'll have to purchase a new "Full Version" of Windows XP Professional
and perform a "Repair Install" using the new Windows XP Pro CD and Product Key
that comes with it. (You cannot simply change the Product Key)
You need to purchase and use a _legitimate_ full retail copy of
WinXP Pro to perform a repair (a.k.a. in-place upgrade) installation,
using the new CD and Product Key.