I recently took an About.com "course" on network security which you can sign up for here. I first found it from another website which lists interesting sources for free computer technology education:
To use the "course" you simple sign up for the "newsletter" then, every day you recieve the next part of the "course". Each part has a couple pages on various things relating to networking and keeping you computer secure. It talks about the TCP/IP protocol, viruses, worms, trojans, firewalls, antivirus programs and more. If you don't know what any one of those words mean, I suggest you take the "course". Even if you do consider yourself a little bit of an expert, it can still be slightly informative.
There are a total of 10 parts, and after each you can take a small quiz. At the end you can take an "exam" which tests you on everything you have learned (or not) throughout the "course".