The key gives you the ability to install Windows 7 legit on the PC, but that download site I wouldn't trust. Lord knows what might be bundled in the .iso file.
Do you have any access to a Windows 7 installation disk from any friends? You already have a key, so it sounds like you just need a disk. If you could get one from the manufacturer like shreader suggested, that'd also be great.
I'd just be really careful downloading Windows 7 from anywhere online... I just think that may cause more trouble than good, one way or another.
"Greatness is not a function of circumstance. Greatness, it turns out, is largely a matter of conscious choice, and discipline." - Jim Collins