Why Can’t I Reinstall Old Copy Of Win 7 On New PC?

Charles from Las Vegas, NV writes: OK, I buy a new machine, and it comes with Windows 8, and I want to go back to Windows 7. If I already own a full copy of Windows 7, why can't I just reformat and install the copy I have and not have to worry [or pay] for any Pro copy of Windows 8? - Chuck B Answer:...