If you have 2 hdd's you could always install another version of windows on that, then when you boot up you will be asked which version you want to load (which is also known as a dual boot system)
The other option is to put your preferred version of windows in the cd drive then restart your pc...
Have to say I find 8 the best version so far, although it is more for tablets with the layout, I find everything runs at about 15% faster on my system. Also all games I play have also had a 20% frame rate increase with my quad sli setup.
Over all I would never go back to the other versions.