http://en.wikipedia.org/wiki/Windows_Virtual_PC#Windows_XP_M...
Apple's notable for having made 3 major transitions (68k to PPC, MacOS to OS X, and PPC to Intel), but all three of those were as minimal as they could be given the circumstances. The architecture transitions were made with almost completely transparent binary compatibility and minimal changes needed for source code, while the source-incompatible OS/API change was made as gradual as possible with a long grace period (which a few companies chose to exploit rather than update their apps before it was too late).
Windows as a system is clearly not clean enough at the moment to easily switch architectures without needing a lot of complex compatibility stuff thrown in. If they try to force developers off old APIs and introduce a new architecture, that new architecture will be a second class citizen and will need a lot of external factors to give it traction.
Windows currently runs on two radically different architectures...x86/x64 and Itanium. It has for more than nine years.
http://en.wikipedia.org/wiki/Itanium#Architecture
http://en.wikipedia.org/wiki/Windows_XP_editions#Windows_XP_...
http://blogs.technet.com/b/windowsserver/archive/2010/04/02/...
Although the article indicates that support will be rolling around for a while.
And lets not forget i860 and the Intergraph Clipper.
http://technet.microsoft.com/en-us/library/cc772344(WS.10).a...
I don't see how Microsoft will ever shed the x86 legacy in Windows. Intel would have an interest in killing off arm-based laptops with aggressive pricing, and there really isn't any history of Windows developers rebuilding apps to support other chip architectures.
Clearly the NT kernel is quite portable. There's plenty of evidence of that (MIPS, PPC, Alpha, IA-64... ). But there's no evidence that even the most modern and supposedly portable components of the desktop experience are actually at all portable. Hardly any of it has (as far as we know) actually been ported.