I personally wouldn't trust that site, if you're a college student you can get Win7 for $30.
Its an upgrade, but I've heard of people being able to use it as a regular install.
If I'm wrong someone correct me.
1) For some people, you just install, and you can activate it, and doesn't even ask you for a previous install. This is what happened to me.
2) If it does ask, you can install the upgrade again. Believe it or not, the previous, inactivated version of Win7 Upgrade counts as a previous install, so the upgrade install after that works. This, by the way, is perfectly legit via MS.