3. In the right pane, right-click OOBETimer, and then click Modify.
4. Change at least one digit of this value to deactivate Windows.
5. Click Start, and then click Run.
6. In the Open box, type the following command, and then click OK.
7. Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
8. Click Change Product key.
9. Type the new product key in the New key boxes, and then click Update. If you are returned to the previous window, click Remind me later, and then restart the computer.
10. Repeat steps 6 and 7 to verify that Windows is activated. You receive the following message:
Windows is already activated. Click OK to exit.
11. Click OK.
12. Install Windows XP SP1 or a later version of Windows XP. If you cannot restart Windows after you install Windows XP SP1 or a later version of Windows XP, press F8 when you restart the computer, select Last Known Good Configuration, and then repeat this procedure.
There are many softwares available which change the product key for Windows. Some of them are listed below.
Windows XP Product Key Modifier Download Windows XP Product Key Modifier from here. It’s very easy to use, simple interface. You just have to enter the new product key of Windows XP and you’re done.
The magical Jellybean Key Finder Download the magical jellybean key finder from here. After unzipping, run the application. It’s interface is also pretty self explanatory. It will show you your current product key. If you want to change your Windows XP product key, go to Options and select Change Windows Key. If you want to alter your Name and Organization, go to Options –> Change Windows Reg info.
If you find any other option to change the Windows Product Key, please let me know.