If you do a free "upgrade" from win7/8.1 to win10, you do not receive an activation (product) key, and no amount of digging will uncover one. M$ determines your computer is allowed to use Win10 by memorizing it's hardware configuration and storing this information on their servers, then creating some sort of secret key code from that. If you change your hardware, you're done. If only you had a product key...but wait! M$ will gladly give you a product key for your "free" "upgrade" for the low price of $100