Beginning with Win8 OEM computers have the necessary key included on the motherboard, no need to enter a key for the version that is included. After setup and connection to the Internet it will be activated or check System to see if it is. Win7 and earlier needed the Product Key. I haven't installed a custom/self-built with Win8 and later using a purchased product key. If the computer came with Home but Pro is wanted it will require a new key. Same if wanting to go from Pro to Home [but no advantage in doing that].