How to change the product key on a Windows XP Computer?

Because of changes in Windows XP Service Pack 1 (SP1), Windows XP-based computers that use a leaked product key that is known to be available to the general public may not be able to install SP1 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 SP1:

326904 Error Message: The Product Key Used to Install Windows Is Invalid


The easier method to use is the Use the Activation Wizard method. Use this method when you only have a few computers on which to change the product key. The Use a Script method is best when you have a number of computers on which you have to change the product key.

Legal Note: Make sure you understand that this method is indeed legal. This is NO crack, NO hack, there is nothing illegal with this article. I will not, however, encourage any reader to use an illegally obtained CD Key. What you do in your own house/office is your own business, and I cannot and will not be held responsible for your actions.

Method #1: Use the Activation Wizard

If you have only a few volume licensing product keys to change, you can use the Activation Wizard.


Warning!
This document contains instructions for editing the registry. If you make any error while editing the registry, you can potentially cause Windows to fail or be unable to boot, requiring you to reinstall Windows. Edit the registry at your own risk. Always back up the registry before making any changes. If you do not feel comfortable editing the registry, do not attempt these instructions. Instead, seek the help of a trained computer specialist.
Note: Microsoft recommends that you run System Restore to create a new restore point before you complete the following steps:

Click Start, and then click Run.

In the Open box, type Regedit, and then click OK.

In the left pane, locate and then click the following registry key:

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

In the right pane, right-click OOBETimer, and then click Modify.

Change at least one digit of this value to deactivate Windows.

Click Start, and then click Run.

In the Open box, type the following command, and then click OK.

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

Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.

Click Change Product key.

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.

Repeat steps 6 and 7 to verify that Windows is activated. You receive the following message:

Windows is already activated. Click OK to exit.

Click OK.

Install SP1 for Windows XP.

If you cannot restart Windows after you install SP1, press F8 when you restart the computer, select Last Known Good Configuration, and then repeat this procedure.