As for @SteveRichter: question, I don't think so. WinRT is win8, not a layer on top of win8. You maybe able port it to .Net and run on win7. But the security would be using .Net instead winRT model.
WinRT is lots of different things depending on who you talk to, but then "kernel" means different things depending on who you talk to as well, so it's all semantics one way or the other.
WinRT classes can usually be used on the desktop, but obviously since you're on the desktop and not on Metro, you can't use WinRT classes designed for interacting with Metro.
You can also use Win32 in metro btw - LoadLibrary and so on are all still there - after all, it's all syscalls and memory operations under the hood - and Metro can do both of those