I can't even begin to imagine how difficult it will be to deal with Microsoft's code for Windows. The sheer volume will be almost impossible to deal with. Further, by this time, the code will be so full of kludges and inactive code that it's going to cause enormous confusion in anyone who tries to decipher it.
I went back a while ago and looked at the source code for one of my old Windows apps, thinking I might get back into the business and update the application. I wrote this thing myself, with no other programmers involved. It's been 10 years since I finished the last version.
After several hours of study, I decided that I'm definitely done with coding. At one time, I could go back into the code and work with it, but I knew it then like the back of my hand, and could keep virtually all of the variables and routines in my grasp.
Windows, since the first version, contains so much code, written by so many people and teams, that it's going to be impossible to comprehend. Microsoft isn't all that terrific about transparency in its code, and I've worked with some Microsoft-originated stuff. Further, their need to maintain backwards compatibility (all the Win 3.1 apps I wrote back in the 90s still run perfectly in XP) means that there is lots of old crap in the current versions.
Good luck!
Do you mean win32?