I thought Ms downloads had a download manager where you could resume an interrupted download -- obviously not it seems.
However presumably you must have HAD an OS in the first place on some sort of computer to have downloaded the W8 Preview drom somewhere. - or please explain to me how you did the original download.
Just re-install that OS - even if it's Windows XP to download your ISO. Just install the base OS - don't worry about drivers or anything - all you want is to have a working copy of IE so you can download the new ISO.
You won't have to activate it (Your old OS I'm referring to here) -- in the worst case you'll get a nag screen saying you have to activate within 3 days -- but if your download takes 3 days I'd change your ISP. !!!