lately i was wondering about the site digital river, and came across an article which said that i can freely download windows 7. but how does that work? do i have to purchase the product key from them afterward? and is this website actually legal or is it some bs?
I don't know anything about the site you mention, but every legal copy of Windows requires a valid product key, and I don't know of any way to get a valid product key except by purchasing it in one form or another.
The only exception I'm aware of is that Microsoft sometimes gives away limited quantities of promotional copies of its OS with valid license keys at some of it's events - for example OS announcement events.
Any non-Microsoft company claiming to give out free Windows product keys is virtually certain to be bogus.
Those sites are simply ISO downloads for people who bought keys legit online from Microsoft and needed a different download link for there copy of Windows. Those sites give a copy of the operating system but not a key to run the system past the 30 free evaluation days. You can go to the Microsoft online store to purchase a legal key.