Hey tech champs >> here's a cool development that can save you up to $80 (often the price difference for a upgrade vs full install disc)
Paul Thurott over at the SuperSite Blog has worked out how to perform a clean Windows 7 install using the upgrade disc.
Normally, you can boot and run full setup on with the Upgrade media for Windows 7(even on a blank drive or partition), but when you go to activate, it won't work.
The workaround instructions are as follows:
1. Perform a clean installation using the Upgrade disc.
2. Make sure there are no Windows Updates pending -- if there's an orange shield icon next to the Shutdown button in the Start Menu, that means you need to reboot before attempting the next steps.
3. Open 'regedit.exe' from the Start Menu -- just type it in there, where it says: Search programs and files. Accept the User Account Control dialogue that pops up to give yourself Windows 7 superpowers.
4. Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/
5. Change MediaBootInstall from '1' to '0'
6. Open the Start Menu again and type 'cmd' -- use ctrl+shift+enter to run it as an elevated-permissions user
7. Finally, type slmgr /rearm -- and press enter.
Apparently, the install twice method used in Vista also works.
For more info >> source