I should have noted though that while these programs are good, all of them can be disabled by a savvy teenager. I knew how to get around them when I was 13 or so, but I was a giant computer geek. Most kids don’t know how. So, keep in mind that while it’s a good tool, and will limit them considerably, you should alos keep the computer in a family room, like the den or somewhere in plain sight, never let them on when you’re not around and check their histories and search terms.
Also, make it a point to stop by every now and then, basically be incredibly annoying without mentioning the computer, bring them a coke or a snack while they’re on and they’ll get the message that you could show up anytime.
At night, log off and password protect and change the password often.
Voila. Totally sheltered child.