I had the same problem, downloaded and used the product key then later found out it was the 32 bit that I installed on my pc. So I went to my laptop which was already running windows 7 64 bit operating system. Then went to windows.microsoft.com/en-us/windows-8/upgrade-product-key-only. I used the same product key and downloaded the ISO, burned it to disc and did a fresh install on my pc. After it was done I checked it out and it actually worked.
It seems you have to be on a computer running a 64bit operating system when you download from microsoft because it automatically chooses it for you based on your current operating system. So your only option might be finding a trial version of some 64 bit operating system to use for the sole purpose of going to microsofts website for downloading the iso or go to a school or a library anywhere you can find a computer running 64bit just to download the iso.