I can buy Win 7 Ultimate Student Upgrade for 15 dollars (Or even get Professional for free).
And will you be getting a legitimate 25 character product key and a legitimate copy of Win 7?
1 license = 1 machine
1 machine = 1 license
For example, you can't use a friend's key to their Win 7 Pro to install Win 7 on your computer and have the Win 7 validated.