I can see Windows maybe just going to a linux base. But the important part for them to open source would be details for .exe executables. This is something I see them not open-sourcing. You'll be able to run .exes on linux, but only if your on the 'Windows' distro. That is at least my take.
The details of Windows executables are all well documented and open - it's very easy to parse a windows executable and run it. The part that's missing to run it without Windows is the implementation of system calls and system libraries. Without those the executable will run into dependencies that you are missing. This is the whole reason for things like WINE - the executables are easy, but the runtime environment that Microsoft hasn't opened up is hard.
