Microsoft have had a "WIN" project one the go in one form or another for years. "Cairo" anyone? For years, Longhorn was going to be an almost complete replacement of the platform. They've never got it to work though, perhaps because it's always been an engineer driven revamp rather than a UI driven one.

To get something like this done they'd probably be better off abstracting the OS and making it insignificant in the way as web browsers do. Imagine the Windows Phone 7 OS running on laptops and tablets, all silverlight driven and no sign of a filesystem anywhere. Then they could release a version that runs on classic Windows, OSX and linux.