Just came to my mind, if you are interested in learning OS's, you should check how Minix works:
The MINIX 3 Operating System

It's full working operating system (of course there's not so many features and not so much stuff, like in Windows) which has only something like 4000 lines of code (In kernel mode). It's a very very little amount of code, and it's meant to be good for education, so I would recommend you to check it.

It's now in version 3, and it has some new goals, which are not only for teaching. Maybe you would like to check versions 1 and 2 because they are smaller and more easier. Minix is the OS which is used when teaching Operating System Programming in Universities. For example Linus Torvalds (Creator of Linux OS) learned Minix in his OS programming course in Helsinki University.

Minix is licensed under BSD license and you are able to get it's source code for free.