If any version of windows from 95a or newer were to be released as open source especially 98 the wine community would immeditly be able to emulate the entire os environement from microsoft source code legally and of course this means total emulation not just api's.
Also patented file systems such as Fat12,16, and 32 although licensed for use in other products and used in products such as linux installers would be derailed and dissasembled causing microsoft to lose the patent a second time.
There are also a few other patented items and source required technologies that would be released that would reak havoc for microsoft's effort to keep things under wraps. With the ability to recompile a 9x kernel we can run windows on the xbox which is only 90% ibm compatible and only linux and ce have sucesfully been installed.
ALso Apple and other deevlopers of the mac os community would be able to more easilly integrate and persuade users to crossover.
Microsoft even 10 years from now wouldnt do this and in the time they would they wouldnt bother with it.
I hope you get the point that itll never happen in the timeframe wed find use of it.