Windows 8 Pro.
Works faster than 7, definitely recommend it, don't listen to the rumors that its bad its only because some people can't accept change or the fact everything looks different.
It just takes a while to get used to it, but once your passed that, everything is perfect.
Also remind me why you need a start button when there is one on nearly every keyboard?
There is also a legacy start menu if you right click on the start button on the lower left corner.