The absolute cheapest way takes a lot of time, but it's 100% legal, 100% free, and 100% legit.

Find a Microsoft launch event thats giving one away and attend. Of course, this is about as feasable as a layman installing Linux, configuring Wine, and getting WoW to run in an unsupported OS... I'm a fairly technical person and after 3 days of working on it non stop, I abandoned the Linux path for WoW, not because of WoW, but because I couldn't find a clearly documented, easy to follow, way to put Flash on the machine. If I can't do something as trivial as browse the web, I certainly don't want it as my main OS.

I would suggest OEM as the best route for you, the problem with OEM is there is no upgrade, of course, if you didn't plan on upgrading then you are fine.