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In 2001, flash mock-ups were created to showcase what a finalized Windows Longhorn might look like. Many different UIs were created, but only one was chosen to be the model designers and developers would code into Windows development. This particular demo spawned the Plex (the name of the theme used in M3-M4 builds) era of Longhorn development at Microsoft. Subsequent themes used were Slate, Jade, and various forms of Aero.
As you can see in this screen shot, there were massive changes planned for the Windows shell.
Here's another conceptual shot of a Plex-themed Windows Longhorn, circa 2001.
This is a Plex concept that extends even farther back in time than the previous two screen shots, thus showing the humble beginnings of a wild new look for Windows.