Changing license key in Windows (Windows Vista/7 Home Premium/Business/Ultimate, Server 2008 etc.) is easy – a graphical interface is built-in to Windows for the purpose. You may want to edit the Windows license product key, for example, on a pirated copy to enter a new valid serial key you bought. You may also need to re-enter the license key if Windows Genuine Advantage (WGA) validation goes bonkers.
To change Windows license serial key
Go to system properties (Start, right click Computer, click Properties). Under “Windows activation” tab, click “Change product key”. You may have to enter your admin password when Windows If User Account Control (UAC) requests it. Enter the Windows product key and hit Next. Windows will activate online with your newly-entered key, or sometimes give you steps to follow to complete Windows activation with the new activation key.