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In the beginning, there was the sidebar! Early thinking in this area consisted of everything from the Microsoft Research team (via an application called Sideshow) to the MSN and Windows team collaborating on visual mock-ups, like the one pictured above. The sidebar was going to be a centerpiece in Windows Longhorn, giving you instant access to the information you most wanted to see -- everything from contacts, to news, to documents and more.
Although a the sidebar made it into Windows Vista, it was more of an afterthought by then, existing as a separate application that was ultimately unutilized and quickly abandoned.
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.