http://windows.microsoft.com/is-is/windows-8/upgrade-product-key-only
Upgrade Windows with only a product key
If you bought Windows 8.1 or Windows 8, you can download and install Windows from this page using just your product key. You'll have the option to install Windows now, later, or using media with an ISO file.
http://pcsupport.about.com/od/windows-8/a/install-windows-8-usb.htm
How To Install Windows 8 or 8.1 From a USB Device