So if you have old Windows and purchase the Win 8 upgrade offer. Yes you can do a clean install. You would download to USB stick (with 3GB available) or DVD and reboot off that. When install starts you will see the usual options to Upgrade or Custom. Select Custom and you can do a clean install, as well as create partitions and reformat etc. The special download will require that old Windows (XP/7/Vista) is detected on your hard disk.
Create the required shortcuts in the Windows desktop. Move them to
If the new tiles don't appear on the Start Screen Search for the shortcuts using the Search Charm and pin them to the Start Screen.
Microsoft provides a tool called "Windows 7 USB/DVD Download Tool". This tool can help create a windows 7/8 USB install drive.
After installing Windows 8 on my DELL Laitutude XT Notebook (Win 8 installed via DVD) I rebooted and got "NTLDR is missing press Ctr+Alt+Del to reboot" (Windows would not start).
There are several solutions around the internet. For me it was just a matter of unplugging any attached USB devices. Then switching on the notebook it booted into Windows 8 fine.