First thing you need to do is search for a program called XPKey.exe , after you find your new key do this below.
How to change product key in XP with no re-install.:
1. Run regedit and go to: HKey_Local_Machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents, in the right pane double click on
"oobetimer" and change at least one digit of this value. This will deactivate windows. Exit regedit.
2. Choose run from start menu and type in this command: %systemroot%\system32\oobe\msoobe.exe /a to get the activation screen.
Go to the second option which is activate by phone. Click Next.
3. In the new screen choose the option to change product key, and type in the new product key (Corporate one only). You cannot use the FCKGW key.
Do not click Update.
4. Close that window, reboot your system and enter the command in step 2 again for the second time to verify your activation , you should see "Windows
Is Already Activated".