Computer Hope
Computer Hope: Computer Operating Systems
This site contains general information about operating systems as well as troubleshooting help for a variety of operating systems.
Wikimedia
Wikipedia: Operating System
This site contains a Wikipedia description of operating systems.
Yale University
Yale University: Pc Lube and Tune
The PCLT objective is to supply usable introductions, tutorials, and education on technical subjects to the large audience of computer users. These are articles that take technical topics and make them understandable for the beginner.