One of the many remaining questions about the Windows 7 Upgrade media is how, exactly, a clean install will work with this version of the OS. On the Microsoft Store, the company simply notes the following:

If you have Windows Vista, you can purchase Windows 7 Upgrade versions. You can do a clean install (back up your files, clean install, and reinstall your applications) or an in-place upgrade (Windows 7 installs over Windows Vista).

If you have Windows XP or Windows 2000, you can purchase Windows 7 Upgrade versions. But you must back up your files, clean install, and reinstall your applications.

OK, but what does “clean install” mean exactly? And how does the Windows 7 Setup routine ensure that you have a qualifying previous version of Windows before allowing the clean install? Thanks to an email from Chris P., we have the following info from Microsoft (emphasis mine):

In order to install the upgrade version of Windows 7, you must have a qualifying Windows operating system installed and activated.  You cannot install an upgrade version of Windows 7 on a blank hard drive. The installation procedure does not ask you to insert a Windows disc in the drive for verification, the actual qualifying operating system must be installed.

If you do not have a qualifying Windows operating system installed with a genuine license activated, then you cannot use the upgrade version of Windows 7 – you would need a “full version” Windows 7 license.

In summary:

1. A qualifying Windows operating system must be installed.
2. The qualifying Windows operating system must have a genuine license (product key) and it must be activated.
3. To upgrade, boot to the qualifying Windows desktop, insert the Windows 7 Upgrade disc in the DVD drive.
4. When the setup menu appears, select Custom (advanced) to initiate a clean install procedure.