WindowsXP and Windows2000 are based on WindowsNT kernel, ofcourse there have been a lot of changes made to the kernel but core/crux still remains the same. The Shell library has been changed a lot, so has the common controls library, but a lot of the code is still based on NT 4.0.
There is ofcourse the next version of Windows (longhorn) which is supposidly a complete rewrite in C# and will have exception handling built into the OS. Only after Longhorn has been stabilised as a OS can Microsoft think of releasing NT 4.0 source as opensource. Moreover mircrosoft would have to stop support WindowsXP before releasing it as open source(which is not going to happen soon as these are the same folks that would ask MS to keep supporting XP for years to come!). So if you add it all up you aint gonna see Microsoft release NT4.0 source for a couple of years even if they want to! Im not saying that they are willing to opensource it, Im just saying it does not make sense to even ask! There are better things to go after MS(like being open,support for IE and standards) but this is one lame request. I think its just plain naive!
P.S: I have seen/worked with microsoft source code in my previous job for 4+ years.