Reset XP and 2003 Product key

Open Registry.  Navigate to "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WPAEvents\oobetimer" and modify the key.  it's a checksum and it needs to be invalidated.

Next paste the following into Start -> Run
"%systemroot%\system32\oobe\msoobe.exe /a"

Select the second option "Activate by phone" and hit next.

Select "Change Product Key."

Enter new product key and hit "Update."
Close the Activation program and install service pack.