May I ask why you would want to do such a thing? Just add the Start button back (and remove the Start screen) to Windows 8 with Start8 or StartIsBack. You'd reap all the new features of 8 (faster boot time, better search, more secure and stable drivers) but with the Start menu right where you want it. It's the best of both worlds, really!
Do you own a legal, retail copy of Windows 7? The Y500 doesn't come with any downgrade options or discs, so you'd have to buy a disc yourself.