3 - How to Change a Windows XP Product Key

Change a Windows XP Product Key

REGISTRY

HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents

RUN COMMAND

%systemroot%\system32\oobe\msoobe.exe /a

1.Click Start, and then click Run.
2.In the Open box, type regedit, and then click OK.
3.In the left pane, locate and then click the following registry key: HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4.In the right pane, right-click OOBETimer, and then click Modify.
5.Change at least one digit of this value to deactivate Windows.
6.Click Start, and then click Run.
7.In the Open box, type the following command, and then click OK. %systemroot%\system32\oobe\msoobe.exe /a
8.Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
9.Click Change Product key.
10.Type the new product key in the New key boxes, and then click Update.If you are returned to the previous window, click Remind me later, and then restart the computer.
11.Repeat steps 6 and 7 to verify that Windows is activated. You receive the following message: Windows is already activated. Click OK to exit.
12.Click OK.
13.Install Windows XP SP1 or a later version of Windows XP. If you cannot restart Windows after you install Windows XP SP1 or a later version of Windows XP, press F8 when you restart the computer, select Last Known Good Configuration, and then repeat this procedure.

see also http://support.microsoft.com/kb/328874