Where to download Win 8 Iso.

Hi, First, I will start by stating that I do have a key. The problem is that I don't know where I can download Windows 8 legally, while the upgrade program does not work. Even tutorials like http://www.eightforums.com/tutorials/18309-windows-8-upgrade-iso-download-create.html are not helping me, since it just gives me an error, probably because the period where they were selling it at a...