You can use an Vista or Windows 7 upgrade key to do a clean install of their respective operating systems.
How? Do a clean install of Windows. Then run the installer (from the DVD) inside the unactivated copy of Windows you just installed to “upgrade”. Your installation is now marked as an upgrade and you can activate Windows with your upgrade key. Make sure you pick the same version of Windows & architecture (x86 vs x64) you are licensed for both times.
What this means for you? You never have to pay full price for Windows, unless of course, if you can’t wait for the second install.
Additional tip: Create a bootable USB flash drive (at least 4GB) from the installation DVD and use that instead. Each install can then be done in about 10-15 minutes since it’s not using the optical media.
EDIT: Update with an alternative method (seems to be more reliable to me).
- Install Windows as described above (the first time).
- Open the registry editor and change the “MediaBootInstall” DWORD value in HKLM\Software\Microsoft\Windows\CurrentVersion\Setup\OOBE\ from 1 to 0.
- Open an administrative command prompt and run the command “slmgr/rearm”.
- Reboot, then enter your key from the Computer Properties and activate.