Hate Windows 8 already? The Pro OEM edition will let you downgrade as far back as Vis
 

Relax, Gabe Newell. If you buy a new Windows 8 Pro PC and discover that you really do detest the OS that much, you may be able to switch back to an older version -- either Windows 7 or Vista, but not near-death XP -- under the same OEM license. Will many folks want to do that? Probably not, and in any case these so-called downgrade rights are actually only helpful in specific circumstances and they don't come with boxed retail versions of the software. The main practical benefit (albeit still a niche one) is that a manufacturer could technically offer a Windows 7 disc in the box with a new Windows 8 machine and give customers a choice of OS. Alternatively, the manufacturer could install Windows 7 by default (effectively a factory downgrade) and supply Windows 8 Pro installation media so that customers can upgrade for free when they feel good 'n ready. At some point, of course, staring at a redundant UI-switching button is going to grate.

http://www.engadget.com/2012/09/03/w...ngrade-rights/