Does the upgrade need to be done on a PC with Windows already
installed? The license terms for the upgrade require upgrading from a
valid “genuine” version of Windows. Visit
http://www.microsoft.com/genuine/ if you are not certain whether or
not your PC has genuine Windows.
I did this with the Windows 7 upgrade. I could not do it on a blank drive, it had to have Vista on it. However, after Vista was on it (didn't even have to boot up), I was able to select the custom install, delete and format the partitions and install Windows 7 cleanly. Whether this changed for 8 I don't know, but according to the FAQ you have a to have a qualifying base OS.