Change Windows key with use the Activation Wizard

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
If you only have a few volume licensing product keys to change, you can use the Activation Wizard.

Note We recommend that you run System Restore to create a new restore point before you follow these steps.
Deactivate Windows
1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. In the navigation pane, locate and then click the following registry key:
   HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4. In the topic pane, right-click OOBETimer, and then click Modify.
5. Change at least one digit of this value to deactivate Windows.
Reactivate Windows and add new product key
1. Click Start, and then click Run.
2. In the Open box, type the following command, and then click OK.
   %systemroot%\system32\oobe\msoobe.exe /a
3. Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
4. Click Change Product key.
5. 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.
6. Repeat steps 1 and 2 to verify that Windows is activated. You receive the following message:
   Windows is already activated. Click OK to exit.
7. Click OK.
8. Install Windows XP Service Pack 1a 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, try the following steps:
1. Restart your computer and start pressing F8 until you see the Windows Advanced Options menu.
2. Select Last Known Good Configuration from the menu and press ENTER. This option starts Windows by using a previous good configuration.
3. Repeat steps 1 through 8 under "Reactivate Windows and add new product key."
If you can install SP1 or a later version of Windows XP and you can restart Windows, you have resolved the issue. If the issue has not been resolved, try method 2 or see the "Next Steps" section for more troubleshooting resources.

Reference : microsoft.comSource URL: http://nilushae.blogspot.com/2009/08/change-windows-key-with-use-activation.html
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