If you guys want really cheap windows keys, click
here.The cheap ones are OEM keys that go for just under 20 bucks, but you can also get some retail keys for about 40 bucks. I got Windows 7 Pro for 17 bucks. If your key doesn't work right away just use the Windows automagic self service activation and you should get verified. Worked great for me and I've used this about 3 other times for a couple of different builds.
If you pay the extra buck you can make sure that if your key really doesn't work, then kinguin will help you out on getting a refund or getting another key.