There is, of course, always the Java approach: open source everything you can, on the assumption that that is 99% of what's needed, and allow others to fill in the gaps.
I don't know that Windows would run without the licensed code, or at least Explorer. If they didn't bother to write their own ZIP library (or use an open-source one) then they probably licensed a lot of other stuff that one would expect a first-class desktop is to have, making it unusable without a bunch of developers fixing it (when their company would probably buy them a license anyway).
If it was released without the third-party code, then it'd be a research project, which'd be pretty neat but not something most probably expect from "Windows is open source".
Java, when open sourced, couldn't run with purely open code (it relied on some binary blobs Sun provided); it's plausible MS could do similar. There would likely be a fair bit of interest in replacing the binary blobs in Windows.
If it was released without the third-party code, then it'd be a research project, which'd be pretty neat but not something most probably expect from "Windows is open source".