This trick will show you how to change Windows XP product key without re-installing the operating system. This makes it possible to install a new genuine key without the need to format and re-install your Windows XP.
1. Goto start menu, run, type regedit
2. In registry editor goto
3. In the right panel open OOBE Timer
4. Edit/Change atleast one entry there, press OK and close registry editor.
5. In run type the following command
and press enter
6. Activation wizard appears
7. Select activation by phone and in the next screen select the option CHANGE PRODUCT KEY, enter a valid corporate product key, press OK and close the wizard
8. Reboot the computer and type the same command in the Run
9. Activation wizard appears again and it shows the message WINDOWS IS ALREADY ACTIVATED.