Recommended readings (theory):
* Operating System Concepts (the Dinosaur book)
* Microsoft Windows Internals
* Architecture of the Windows Kernel (with Networking)
http://www.facultyresourcecenter.com/curriculum/pfv.aspx?ID=7400
* Computer Organization and Design
There was also another really book, whose title escapes me, which described the network stack and different protocols at each level in the OSI stack. I'll hop down to the library and see if I can find it again, it was a really great book.
Recommended stuff to play around with (practical):
* NACHOS - Not Another Completely Heuristic Operating System
http://www.cs.washington.edu/homes/tom/nachos/
* Windows research kernel - you usually need a subscription, however there was a thread here which described a trick which allowed you to download anything from the microsoft website. Search subject.txt if you're interested.