Totally agree with DBZ. I think MS learned in the 9x days that piracy still furthers their platform. Then in the XP era they got greedy after their popularity and went on an anti piracy storm with their activation requirements. I think by Vista they sort of wised up and realized pirates will still pirate, and honest people will still buy their stuff. So by Vista, you could clean install an "upgrade", I'm sure they knew this was possible, but they just marketed it otherwise. In Windows 7 you can clean install an "upgrade" version too, and I actually think its easier (afaik in Vista you had to install without a key, then 'upgrade' that install with your upgrade key - I don't think you have to do that first install in 7 but I'm not entirely positive).
Luckily for me, I've been able to purchase the educational versions of Vista and 7 for really cheap, I think 7 Pro cost me $29.
In any case. Buy the OEM. If you run into real issues and everything hits the fan, in my limited experience with something like that, a call to the MS activation number gets you fixed back up.