Windows 7 Clean Install from an Upgrade Disk is a post from: Stuffworks.com

Windows 7 is finally out and there has been much anticipation about what version a person should buy to get the best overall Windows experience.  Microsoft is still making it difficult to clean install Windows 7 with Upgrade media, but fear not, there is some good news. While you can’t simply use Upgrade media to do a clean install of Windows 7 on an unused PC with a blank hard drive, the workaround this time is easier than ever. Assuming you know the trick.

If you have ever done this before, you may remember how this used to work? In older versions of Windows, Microsoft would actually prompt you to insert an install floppy or CD from a previous Windows version, to prove that you were qualified for the upgrade version. But beginning with Windows XP, PC makers were able to dramatically change the Windows install disc, so much so that, in some cases, those discs weren’t even identifiable as valid install media to Windows Setup. Clearly a different system was required.

In Windows Vista, Microsoft supported in-place upgrades from Windows XP, but if you wanted to use an Upgrade version of the Windows Vista Setup disc to do a clean install, you had to perform a weird double install trick. This process is documented here at winsupersite.com.

Since Windows 7 is simply a slightly updated version of Windows Vista from a Setup and deployment standpoint, most people familiar with the process could have guessed that the Vista trick would be required in Windows 7 as well.

This is what you don't want to see.

With the general availability of Windows 7 Upgrade media, you can now try this trick yourself. First of all, the double install trick from Windows Vista actually still works in Windows 7. But there’s a better way.

Here’s how to clean install Windows 7 using Upgrade media and a new or reformatted PC with no installed OS.

First, perform a normal clean install of the OS by booting the PC with the Upgrade Setup disc and stepping through Setup.

After performing the clean install, ensure that there are no Windows Updates pending that would require a system reboot. (You’ll see an orange shield icon next to Shutdown in the Start Menu if this is the case).

Then, open regedit.exe with Start Menu Search and navigate to:

HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/

Change MediaBootInstall from "1" to "0".

Open the Start Menu again and type “cmd” to display a shortcut to the Command Line utility. Right-click this shortcut and choose “Run as administrator.” Handle the UAC prompt.

In the command line window, type: “slmgr /rearm”  without the quotes.

Then tap ENTER, close the command line window and reboot. When Windows 7 reboots, run the Activate Windows utility, type in your product key and activate windows. It should just work.

The method described above should work for just about anyone, with the understanding that any pending Windows Updates could screw things up. If that happens, just shut down, restart, rerun the command line sequence, and retry activation.

Please let me know if you have any issues with this or if you discover another method that did work for you.