It is possible to do a clean install from an upgrade disc.
2. Install normally but DO NOT enter your product key:
Make sure you follow this step. Do not select the activate automaticaly option
3. Select the version of Windows Vista purchased:
4. Re-install Vista within Vista:
Once you are finished installing Vista, go to My Computer, and install Windows Vista.
5. Install and type in your Product Key:
When prompted, enter your Product Key.
6. Perform a Custom (advanced) Install:
You will come to a point where you have to choose between an Upgrade and Custom Install. Make sure to choose Custom. Windows should now perform a clean install of Vista on your PC. Don’t forget to restart.