You can also try
Spybot . Easy to use. Just delete items marked in red. It finds dialers very well and is free.
Unfortunately anti-virus software will not stop these dialers. Running Spybot on a regular basis will minimize your exposure. I hope that he didn't use that service. They are expensive and will show up on your next phone bill.
You should get a firewall.
ZoneAlarm is a good one to stop unauthourized intrusions (hackers) or outgoing calls (trojans). Easy to set up and train and is also free. Go to the bottom of the page.
Decent anti-virus is
AVG also free (see a trend?

)
These measures will help to prevent and find crap on your system, but cannot prevent curiosity or stupidity. Ground him for a month with no access to your computer.

That might teach him something.
Regards
augie
<edit>
Here are some programs that you could use but the kids will probably beat them anyway. The best way to control this that I found was to relocate the computer in the family room. If he screams that you don't trust him anymore, remind him that he broke that trust