According to this article, you will need to have an activated copy of XP, Vista, or 7 RC first:
Now here’s the bigger catch: While you
won’t need to downgrade from Windows 7
RC to Vista or XP in order to install
Windows 7 final the first, time, you
will need to install one of those
operating systems and activate it
first if you want to do a clean
install in the future. Because the
Windows 7 upgrade disc will only work
with an activated version of Windows.
That means you’ll need an activated
version of Windows XP, Vista, or
Windows 7 RC to start with or the disc
will be useless.
You can do it.
Paul Thurrott documented atleast 3 different ways to do it:
Method #1: Just perform a clean install
Many, many readers report that they have been able to simply treat the Windows 7 Upgrade media as if it were Full media, and that it just works.
Method #2: Registry hack
- Change in
value of MediaBootInstall from 1 to 0.
- Run as administrator following command
- Start activate utility
Method #3: The good old “double install” method
- Install Windows 7 as custom install on clean hard drive
- Install Windows 7 again now as upgrade
- Activate it
See that link above for more details.
The upgrade versions will not do a clean install onto a blank hard drive. The install process will have to be started from an already installed and activated copy of windows. You will then be able to choose whether the install does an inplace upgrade (vista only) or installs a clean copy of Windows 7 (in which case the old windows files will be moved to a folder called Windows.Old).
I haven’t tested this but it looks like it can be done:
- Performing Clean Install of Windows 7 with Upgrade Disk
- How to clean install from Windows 7 Upgrade media
But whether this won’t be blocked by MS at some later stage is a whole another deal.