How to change XP Serial/License number

Go to / Start Menu/ Run/ Type regedit and click OK. Before any operation in the registry, he should make a backup of the registry as a precaution
In the Registry Editor, click File then Export and save the registry at a place where you can find the file.
Expand the options until you get the key:HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\windowsNT\CurrentVersion\WPAEevents
Then on the right panel, change the value OOBETimer by changing the number.
Close the registry.
Start Menu> Run> Type: %systemroot%\system32\oobe\msoobe /a
Choose Activate Windows by phone, then click Change Windows key and then enter your new key and click Update.

Warning: Take backup of ur Registry before going to edit it.
I dont give you 100% confirmation wether it works or not