In order to use this key, you must enter key into an unactivated copy of Windows 10 Pro or have new fresh clean installation of windows 10 Pro,and also It cannot be used as upgrade key (windows 7/8) keep in mind that this key can activate ONE windows device, only ONE time. After the activation process is done, you will NOT be able to use the same key again. Formatting the hard drive will effectively erase the key. You CAN NOT transfer the license to another computer...