Parental Control / Computer Monitoring Software:
Cyberpatrol lets you monitor and restrict access on your computer - and it's as bullet proof as they come. Most kids (and guests on the computer) will not be able to get around your restrictions unless they figure out your password.
Cyberpatrol offers a two-week free trial.
Also, check out Net Nanny and their 15 day free trial.
Finally, if your system is fast enough, you can use Norton Internet Security, to block access to specific types of sites, although it lacks other features like blocking certain search words and monitoring chats.
For all these program, however, there are ways to get around them if your teen is tech-savy enough. So if you have reason to be extra restrictive, use server-side filtering, like AOL or Arilion.