Microsoft's upcoming Windows Vista will be a technologies-based release, with some UI changes like Aero and Vienna will be targeted directly at revolutionizing the way we interact with our home and office PCs.
For instance, the "Start" philosophy, introduced in Windows 95, may be completely replaced by a "new interface" which was said in 1999 to be scheduled for Blackcomb, before being moved to the Longhorn project, and then back to Blackcomb. The interface involved some ideas based upon truly 3D Windows and an integration of what is now known as Sidebar.More details over here. A release date for Windows Vienna isn't really known but it will take at least a couple more years I guess. If the development of Windows Vienna takes as long as the development of Vista it may mean that we shouldn't expect it before 2010/2011.
The Explorer shell is expected to be replaced in its entirety, with features such as the taskbar being replaced by a new concept based on the last 10 years of research from Microsoft's "Vibe" lab. Projects such as GroupBar and LayoutBar are expected to make an appearance, allowing users to more effectively manage and keep track of their applications and documents while in use, and a new way of launching applications is expected - among other ideas, Microsoft is investigating a pie menu-type circular interface, similar in function to Apple's dock feature.