Changing the product key on Windows XP
For most Windows XP installs, you’ll never need to worry about the validity of the product key assigned to your copy of the OS. However, software does tend to get installed without authorization, even in the most carefully managed shops, and so from time to time you may need to reset the XP product key.

For example, perhaps a user installed a pirated copy of XP but now wants to go legal. Maybe you've been hired by an organization that installed 100 pirated copies of XP but now has a legitimate volume-licensing key (VLK). Perhaps an end user purchased an additional retail license for XP but needs to use his original CD to install the software. When situations like these arise, changing XP's product key is often the most practical—or only—solution.

Determining if you have a valid product ID
Hopefully you already know if you're dealing with a pirated copy of XP. But if you're unsure, a quick way to tell is to install Service Pack 1. Shortly after releasing Windows XP, Microsoft realized that most pirated XP installations were using two specific VLKs, the most popular of which begins with "FCKGW.” These VLKs produce product IDs that match either XXXXX-640-0000356-23XXX or XXXXX-640-2001765-23XXX, where X is any number.

If you try to install SP1 and get the following error message:

The Product Key used to install Windows is invalid. Please contact your system administrator or retailer immediately to obtain a valid Product Key…"

You are dealing with a pirated copy of Windows. For more information about obtaining a valid product key, see Microsoft Knowledge Base article 326904.

You can also directly check the OS’sproduct ID by right-clicking on My Computer, clicking Properties, and selecting the General tab. The machine's product ID will be located under the Registered To section. If the ID matches either of the two models commonly associated with VLK fraud, you’ll need to obtain a valid XP product key before proceeding. None of the procedures described below will work without a legitimate product key.…

How to Change the Product ID in Windows XP

HOW TO: Change the Volume Licensing Product Key on a Windows XP SP1-Based Computer

http://support.microsoft.com/default.aspx?scid=kb;EN-US;326904

http://support.microsoft.com/default.aspx?scid=kb;en-us;321636

http://support.microsoft.com/default.aspx?scid=kb;en-us;328874

http://techrepublic.com.com/5100-6270-5034890.html?fromtm=e103