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During PDC 2003, Hillel Cooperman (Tjeerd Hoek's partner in crime on the MSX team) gave an unbelievably exciting, no-pictures-allowed (though some folks, like Paul Thurrott, were given permission to take and share some pictures at the time) demonstration of Windows Longhorn that was MUCH more ambitious and FAR more visually-exciting than the Plex-themed Longhorn.
Though you can't tell from this screen shot, the background, called Aurora, animated and was to be realized via a then-new markup language called XAML. For the first time, Microsoft was going to make it easy for developers and designers to work together seamlessly -- or so, that was the goal.
Once logging in from the previous screen, the user was sent to this absolutely gorgeous desktop. To note, this presentation was only a mock-up made and demonstrated in Macromedia Director (flash-based, basically). This look would directly inform visual development of Longhorn straight up through its cancellation with build 4094.
Though the Aero name and some of its properties (like transluscency) would carry on through the completion of Windows Vista, the lack of many of the shell changes that would have truly shown it off made it nothing like its earliest incarnation, as showcased throughout the next handful of screen shots.
The Start menu in this Longhorn concept was animated in numerous ways: the Windows flag waves, there's an aurora animation running on the left-hand pane, and as you hovered over menu items, their respective icons would swing into view at the top of the left-hand pane. It was an awesome, awe-inspiring vision of the Start menu!