i had the exact same problem last saturday and i deleted all traces of NF from my computer ( also windows7).
As a last resort before smashing my keyboard i logged into windows under a different user account, in my case i logged in as guest, and dowloaded the game one last time and tataaa...it worked. after this my install on the admin user account started to work again too.
Not sure what the issue was, but it has been fine since.
Maybe give this a try