So I have legit keys for two version of Windows 7 Professional, but I am running on a 32-bit machine right now and wanted a fresh install of Windows 8 Professional 64-bit. So I take one of my spare partitions and wiped it and installed Windows 7 Professional 64-bit on it, because the Upgrade Assistant won't let you upgrade from 32-bit to 64-bit, and well, I want a fresh install anyways. I got a legit ISO from My Digital River (the host that MS uses), and made a point of NOT entering my legit Windows 7 Professional key, just to see what would happen.
I then downloaded the Upgrade Assistant for Windows 8 and was at the screen to buy it for $39.99...but then I stumbled upon a webpage that allows you to get Windows 8 for $14.99 if you recently bought a PC with Windows 7...this site does not ask for any serial numbers or anything, which is strange.
https://www.windowsupgradeoffer.com/
Fill out the info, with ANYTHING YOU WANT (I entered legit stuff), and it will send you a promo code. I entered this at the screen in the Upgrade Assistant where you finalize your purchase, and it worked and instantly applied! I then made the purchase and was given a fresh Product Key.
tl;dr version: Anyone can download a legit digital copy of Windows 7 for free, fill out the information on the site linked above, and basically buy Windows 8 for $15. I'm downloading Windows 8 as we speak.