From Micosoft:

Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 (http://support.microsoft.com/kb/256986/) Description of the Microsoft Windows registry

Warning The steps in the article are effective only on Volume License media. If you try these steps on OEM media or on retail media, you will not change the product key.

INTRODUCTION

loadTOCNode(1, ‘summary’); If you use a “leaked” product key (a product key that is known to be available to the general public) for the deployment of Microsoft Windows XP across multiple computers (a Volume Licensing installation), you may not be able to install Windows XP Service Pack 1 (SP1) or later versions of Windows XP or automatically obtain updates from the Windows Update Web site. For example, you may experience the symptoms that are described in the following Microsoft Knowledge Base article when you install Windows XP SP1 or a later version of Windows XP:

326904 (http://support.microsoft.com/kb/326904/) You receive a “The product key used to install Windows is invalid” error message

This article describes how to change the Windows XP product key after a Volume Licensing installation. You can use the Windows Activation Wizard graphical user interface (GUI) or a Windows Management Instrumentation (WMI) script. The Activation Wizard method is easier, but if you must change the product key for multiple computers, the script method is better.

Note Microsoft recommends that you run System Restore to create a new restore point before you follow these steps. For information about how to create a restore point by using System Restore, see the “To Create a Restore Point” help topic in Help and Support.

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.

For additional information Click Here.

[tags]Windows XP, product key, change, registry, backup, restore, [/tags]