It appears that incomplete segments of Win2k and WinNT source have been leaked to the internet. In fact, if I read this correctly, Micro$oft even admitted that this is true.
If your eyes are glassing over at the sight of another technical post, let me spell out what this means to you. Bear in mind that there are already a LOT of known Windows insecurities which allow crafty hackers and programmers to write programs which bork Windows. Also bearing in mind that WinXP is about 95% just Win2k repackaged, this is kind of like releasing a large segment of the map to Fort Knox. Before, people just had to walk around the outside of the wall looking for cracks. Now, people can pore over the map and find them a whole lot easier and faster. This should be fun...
Update: Microsoft has issued a press release admitting that there is, in fact a leak. The BBC has an article, essentially noting that MS and the major players in Windows security are saying "wait until we see what's up." That said, this article off of Reuters is noting that there is 200+ megs each of Win2k and WinNT source. While that is under 5% of the code, that's still a lot of information.
Long story short, there are a couple of code segments out there. We're just not sure what all they entail. Give me another 24 hours and we'll they'll have more information on this