Click "Start" and go to "Run." Enter the following string: %systemroot%\system32\oobe\msoobe.exe /a
Click "Yes, I want to telephone a customer service representative to activate Windows" and then click "Next."
Click "Change Product Key."
You will now be able to enter the valid license serial number for your Windows XP.
Press "Update" and close the window.
Restart your computer. Your Windows XP version now has its new serial number. Just do this to the rest of your computers if you own a computer shop.