Hey there!
I've been asked to explain, how I seperate Testing- and Gaming-Windows-Installation from my productive system. No problem. 
There is more than one way. So grab the best for yourself.
Reasons for doing so
Sometimes software's doing nasty things to your Windows-Installation. DRM-Solutions running wild, drivers will cause problems with other hardware and so on. When you only do software-testing (No drivers!) a virtual machine will do fine. But when you want to play games or check new drivers you can't use a virtual machine.
Easy Hardware-Solution: eSATA
The easiest way is to add some hardware. Buy an eSATA-Controller (Example) and a proper eSATA-Dock (Example). When you install the controller to you PC disconnect for a moment your internal HDDs and leave the housing open. Now plug the dock in, add an HDD to the dock, switch your PC on and install your favorite Windows. Now Windows will be installed to the one and only HDD you've plugged in. This means, the bootloader is included to this HDD. After that, shutdown Windows and plugin all your internal HDDs. Now boot your "external Windows" once again. That will now recognice the internal HDDs and will mount them. That is not wanted. So go to the "Disk Management Snap-in" and set the internal HDDs to offline. Now you're done. I would recomment to do an image of that installation. So you save time in case of a new installation. (Hint for the german readers: c't-WIMage looks pretty nice! Never tried it because I don't have Win8.1. English readers: It damn new. So maybe it needs some time to get translated.)
My daily use: If I not want to play a game or do testing, I just leave the dock powered off. My PC starts to the productive system. If I want to play a game, I reboot my PC, power on the dock, hit the POST-Key for my boot-menu (depents on mainboard) and boot my "external Windows".
The trick: When you're using eSATA, windows thinks, it has been installed to an internal HDD! 
More tricky Hardware-Solution: Win-Installation on USB-Drive
I never tried that, so I googled one HowTo for you: Klick! Like said: This is more difficult to do. The eSATA-Solution is so easy, so I never tried the USB-trick.
Tricky but cool Software-Solution: Boot to VHD
This is although used by me. I'm running this on Win7 Ultimate. It works only on Ultimate- and Enterprise-Editions and not on other Editions (Licence-Ristrictions). Win8 would work too, but I never tried it. The idea is, to copy your clean Windows-Installation to a virtual disc (VHD). Than you nail that down and create a difference-VHD, that you're booting to. If you're diffenrence-VHD dies, you can delete it, take a new one and test once again. So pretty cool. Because of the limitations to Win7 Ulimate and Enterprise, I just give you one example-link, which seem to explain that in english language. If you can read german-language, that is the way I did it: "Eingebauter Blitzspiegel".
