What is the best way to track what my teenage son is doing on the internet?

I guess this is what you're looking for

[PC World - PC World Downloads - Computer Use Reporter

Hope this helps -- Please rate my answer

Create a separate Windows user account for him and look at his history and cache.

Do not allow use of the computer outside of the common area of the home, if you are sitting there watching what sceens come up, then nothing you should not see will come up.

Check out these sites -- Hope you find something that works

FREE Parental Control Software & Internet Filtering

K9 Web Protection - Free Internet Filtering and Parental Controls Software

free parental control software - Google Search

Let us know how you deal with this issue -- I'm sure more people share this problem

Let me tell you, K9 works. I have it installed on my computer. I got it with a subscription to Computer Associates.