>"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."
Windows currently runs on two radically different architectures...x86/x64 and Itanium. It has for more than nine years.
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.
Wow, that's a sad list. It looks like they didn't even port half of the server components to the Itanium, and they warn you not to use .NET for anything performance-sensitive. The closest thing they've got to a modern, portable environment and their first port of it is so embarrassingly slow that they have to warn you on the feature list. Even worse, their modern GUI APIs (WPF) weren't ported at all, so even simple .NET apps are at risk of not working due to not using an archaic GUI toolkit.
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.
There's no reason all of that stuff couldn't run on Itanium too, but it's a matter of choosing what to invest in; OS releases have a huge test cost with supporting anything, and Itanium is too small a market - ARM is different because it's clearly consumer-based and it's here to stay. I assure you, it is all portable.
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.