When you're about to install Windows 8 using its upgrade disc, you're supposed to have an older version of Windows on the disk. Its presence will be noted and you can format the disk to make a clean install, or you can upgrade it which I prefer not to do. When you try it on a completely clean disk, Windows 8 will still install, but you won't be able to activate it due to the missing previous OS.
Top tip: If you ask nicely, it will allow your otherwise legal, purchased copy to activate.
Modify this key in the registry from 1 to 0:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ Windows\CurrentVersion\Setup\OOBE MediaBootInstall = 0
Then run this from command line as Administrator:
Reboot and you're done; you can activate your copy of Windows 8.