Here's a good way to learn C, jus read the DOS sourcecode.

Description: The essential part of the operating system, responsible for resource allocation, low-level hardware interfaces, security etc.
http://www.planet-source-code.com/vb/scripts/ShowCode.asp?lngWId=3&txtCodeId=3605

Keep in mind for all that doesn't know that the DOS kernel was stored in the MSDOS.SYS (which is no longer the case as it is now a mere text file with a few configurable options) and NOT the command.com.