In the past, there were 3 important leaks of Microsoft source code.
First, it was Msdos_6.0_source_code.rar. 20MB
Then there was windows_nt_4_source_code.zip. 230 MB.
Next there is w2sc.zip which is Windows 2000. 210MB.
The source code reveals the complexity of the Operating system and of all its small programs that made what we call Windows.
I study the internals of Windows since 10 years and it’s still fascinating me. The complexity and the beauty of the code is amazing.
As a COM love (like Don Box said), I study all the COM infrastructure.
Recently, I had access to the Windows Research Kernel (WindowsResearchKernel-WRK.zip) which represent to XP 64bit/Windows Server 2003 32bit code for the University track.
There is just the kernel into the package and it is available on University under NDA. To study the kernel, that’s perfect. The book Windows Internals is provided.