Windows 8 PCs now keep their product keys in the BIOS, a move that offers both pros and cons.
In the past, a new Windows PC would display its product key on a sticker, usually on the side of a desktop and on the base or the bottom of the battery compartment on a laptop. But with Windows 8, Microsoft has switched gears and now stores the key as a BIOS setting instead.
Read more at source:
Windows 8 moves to BIOS-based product keys | Windows 8 - CNET Reviews
This is only for newly built OEM machines that come preloaded with Windows 8.