I had good luck with Cybersitter:
http://www.pcmag.com/article2/0,2817,1618830,00.asp
It lets you pick and choose what content you want to allow/disallow, you can disallow internet completely while still allowing computer use, it has a timer, so if I wanted on the computer, I could disable the software and set the timer to turn it back on when I was done (so I didn't have to remember to turn it back on), and it kept my computer-savvy son from getting into trouble on the computer for a year. It would have worked longer, but he was able to get my password for the software, and by then he was old enough that I just gave up. Some of the reviews state that it caused people to have problems with their computers, but the only problem I had was when I tried to buy something online and had forgotten Cybersitter was on (one of the controls I chose was to keep my son from being able to purchase anything on the internet).
I found a link that compares several different software packages that might be helpful:
http://internet-filter-review.toptenreviews.com/
