When will a Windows cygwin version be available/stable

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A cygwin version is now available from the Download section of the sagemath website thanks to Erik Bray.

This question is fine. The best answer I can give you is "whenever enough people have time to make it that way". There are three-five people who have been giving it effort over the last several years, but there is not a dedicated resource for doing this, and none of the ones who are doing it have Windows as a native system (I run it in Parallels, for instance).

The current status is that Sage builds fine every time except Maxima (esp. on Windows 7), and there is at least one bizarre segfault on startup. In addition, even when Maxima builds, for some unknown reason we get an importation error to use it for calculus (Maxima itself works fine using ./sage -maxima).

But Sage has built and run in the recent past, with relatively few errors (mostly in fairly technical code for elliptic curves), so these are certainly surmountable obstacles. WHEN they will be surmounted is hard to say. There simply aren't enough people willing to try building and testing (which, on Windows, does take some time) who also have the know-how to fix these issues.

As to your more immediate problem, I suggest using Sage via a notebook server, or perhaps trying out the "LiveCD" version, though that doesn't seem to have been updated for a while. I assume the VirtualBox solution was not adequate for you? It isn't ideal, it's true.