Change 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.”