I had the same problem with Windows 7. I had the Windows 8 release preview installed on another disk and used that and whammo, it started to download immediately, no idea why.
Try rebooting if you cant still get it, sometimes that helps, maybe there are just too many people downloading.
do u have proper internate connection?if you see a yellow a mark over internet access icon in taskbar,then it may not work.because i tried when there was similar mark over internet access symbol.later i tried without any symbol over the icon. my download progresses.may be it help u.reply