I just discovered this happened a while ago in 2004 when i was randomly googling "decompiling windows" to see if anyone had ever tried for ****s and giggles.
The main report here, and a rough examination of the code
here.
Not much was learned, microsoft worries a lot, and some rather big statements were made that i find can't be proven since only 15% of windows 2000 source was leaked.
Everyone knows that microsoft has made big sacrifices for backwards compatibility (didn't need to examine source code to see that). Backwards compatibility is not a bad thing to strive for, they could do it better. Microsoft does rely on security through obscurity as part of their security model. That based on how secret microsoft source code is and how much they were worrying when windows 2000 source code got leaked.
Thirdly, maybe microsoft did this on purpose. This particular code being made public isn't very damaging for microsoft to have released. People who examined the code noticed no links back to linux. The only link to a form of unix was when microsoft used some bsd code in the tcp/ip stack in windows (the bsd tcp/ip stack was legal though based on what the bsd license lets you do).
In which i find all too convenient for some random windows source code to be leaked that's sort of useless to have. A great public showing of how microsoft doesn't steal code or ideas from linux and unix. That's a bold statement to be having with only 15% of windows 2000 source code that was released. A statement that was only 15% proven. Microsoft makes bold unproven statements all the time. They are the boy who constantly cries wolf in patent infringement and intellectual property, just that there never will be any wolf. I think they did this whole thing on purpose. After all, the juiciest 85% of the rest of it with all of the application data for big things like internet explorer, mspaint, and notepad, etc are not included; no one gets to see that portion of the code and hold it up against the bold statement microsoft made that they didn't steal code.