Here are the steps you need to do in order to change the product key of the windows XP :-
>> First of all create a windows restore point, as we are going to edit the registry, so for the safer side it’s recommended to do that.
>>In windows XP if you want to change the product key, First of all you need to deactivate the current windows.
>> Open “Run” box and then type “regedit”, hit enter.
>> Go to the location HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsNTCurrent VersionWPAEvents
>> At this location you will see “ODBETimer”, right click on it and select modify.
>> A new box will pop up with many numbers you need to change any one of the number an this will change the correct product key and thus your windows will be deactivated.
>> Now open the “Run” box and then put “C:windowssystem32oobemsoobe.exe /a” and hit enter.
>> Now the Windows Product Activation Wizard will come up, here you need to select the second option ““Yes, I want to telephone a Microsoft Customer Service Representative to activate windows””
>> Now you will get the option to update the product Key.