This tutorial explains how to change your windows xp product key / xp serial / xp license without using software or reinstalling. To do this you need to edit the registry to deactivate windows, then run msoobe.exe to enter your new product key.
You can download software to help you do this, but that’s for girls. Real men do it this way…
1. Run regedit and drill down to:
HKey_Local_Machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents.
Select WPAEvents then on the right double click “OOBETimer” and change at least one digit of this value to deactivate windows.
2. Choose run from start menu and type this command in the run box:
%systemroot%\system32\oobe\msoobe.exe /a to get the Activate Windows wizard, select “Yes, I want to telephone a customer service representative to activate Windows”, click next.
3. In the next screen click the “Change Product Key” button.
4. Now enter your new product key, click the update button, close the wizard and reboot. Please note the XP product key FCKGW-RHQQ2-YXRKT-8TG6W-2B7Q8 in the image below will not work. This is the “corporate” key used by pirated versions of windows and has now been blacklisted by Microsoft.
5. When your computer restarts,  enter the command in step 2 to verify windows is activated and you should see “windows is already activated”.
Changing your xp key is perfectly legal if you have a valid key. You can even automate the process with a visual basic script which you can find on Microsoft’s website.
Update: If you click the link to Microsoft above you will now find the following message:
Warning The steps in the article are effective only on Volume License media. If you try these steps on OEM media or on retail media, you will not change the product key.
But unless something has changed this just isn’t true. I have used the visual basic script they provide to change both OEM and retail media keys and it worked just fine.