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 authorisation, 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 organisation 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 realised 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's product 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. Two methods of changing Windows XP's product key
You can change a Windows XP installation's product key either by editing the registry or by using one of two Windows Management Instrumentation (WMI) scripts. The registry editing method is outlined in Knowledge Base articles 321636 and 328874 and works on Windows XP Home, Windows XP Professional, and Windows XP Corporate Edition. The script method is outlined in article 328874 and is designed to work on Corporate Edition installations that use a VLK and do not require activation. It may work on a Home or Professional installation, but I have not tested this scenario. The script method is the practical solution for changing the product keys on a large number of machines. Regardless of the method you choose, make sure to backup important data before changing a product ID, since an unexpected problem could render the machine unbootable and necessitate a complete reinstallation of Windows. Warning
The following instructions involve editing your system registry. Using the Windows Registry Editor incorrectly can cause serious problems that require the reinstallation of your operating system and possible loss of data. ZDNet UK does not support problems that arise from editing your registry. Use the Registry Editor and the following directions at your own risk. Editing the registry
Begin by opening the Registry Editor and navigating to

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

In the right pane, right-click the ODBETimer binary value and select Modify. Change at least one character of this value to either a number from 0 to 9 or to a letter from A to F, then click OK and close the Registry Editor. This renders the current product key invalid and deactivates Windows. Now, it's time to reactivate Windows using your new product key. Click Start | Run and enter the command:

%systemroot%\system32\oobe\msoobe /a

where %systemroot% is your Windows directory. In many cases, this command will look like:
C:\windows\system32\oobe\msoobe.exe /a

At this point, Windows will launch the Product Activation Wizard.