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Microsoft Knowledge Base article Q321636 contains:

This article describes how to change the Windows Product ID for a current installation of Windows XP.

If you acquired an additional license, you may have to change the Product ID to activate a second installation of Windows XP by using your original CD-ROM. If you installed Windows XP on a second computer by using the original CD-ROM, and then purchased another copy of Windows XP, you may want to change the Product ID and then activate the new installation, instead of reinstalling Windows XP.

See the "More Information" section of this article for details about the purchase of an additional license for your retail version of Windows XP.

MORE INFORMATION

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
When you acquire a new Product ID for your Windows XP installation, follow these steps to change the Product Identification number:
  1. Start Windows XP.
  2. Click Start, and then click Run.
  3. In the Open box, type regedit, and then click OK.
  4. In the left pane, locate and then click the following registry key:

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

  5. In the right pane, right-click the OOBETimer binary value, and then click Modify.
  6. Change at least one digit of this value to deactivate Windows.
  7. Click Start, and then click Run.
  8. In the Open box, type the following command, and then click OK:

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

  9. Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
  10. Click Change Product key .
  11. Type the new product key in the New key boxes, and then click Update.
  12. If you are returned to the previous window, click Remind me later, and then restart the computer.
Product Activation works by confirming that the software product key or Product Identification number (required as part of product installation) has not been used on more computers than is permitted by the software license. Activations on the same computer that use the same product key are unlimited. Product Activation discourages piracy by limiting the number of times that a product key can be activated on different computers.

For more information about Windows Product Activation, visit the following Microsoft Web site:

http://www.microsoft.com/piracy/basics/activation/mpafaq.asp#policies

For more information about how to purchase a second license for your current copy of Windows XP, visit the following Microsoft Web site:

http://www.microsoft.com/windowsxp/pro/howtobuy/pricingretail.asp

NOTE: You can also purchase an additional license for a retail version of Windows XP by calling (888) 571-2048 (United States only).

For additional information about Windows Product Activation, click the article numbers below to view the articles in the Microsoft Knowledge Base:

302806 Description of Microsoft Product Activation

302878 Frequently Asked Questions About Microsoft Product Activation