How to change the product key on a computer that is running Windows XP

For whatever reason your computer may have an illegal copy of Windows XP installed on it. But then you buy a legit copy, and would like to use your product key. Instead of formatting the system, you can make the switch using the following procedure. Please note that this will work with Windows XP that has SP1 (service pack 1) and above. Also, if you don't have any experience using the registry editor, I don't recommend this for you.

Deactivate Windows
Click Start, and then click Run.
In the Open box, type regedit, and then click OK.
In the navigation pane, locate and then click the following registry key:
HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
In the topic pane, right-click OOBETimer, and then click Modify.
Change at least one digit of this value to deactivate Windows.

Reactivate Windows and add new product key
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 1 and 2 to verify that Windows is activated. You receive the following message:
Windows is already activated. Click OK to exit.
Click OK.

This is a legal method of changing your product key. The method was written by Microsoft. To read the original Microsoft support article visit http://support.microsoft.com/kb/328874