How to stop porn in computer?

[Xmasgee]

My stepdad always watches and downloads porn and ruining our computer. Every time we get a new computer he has to ruin it in a way that it's fubar. Also, My mom knows and so does everyone in the house. He only does it when hes wayy drunk and doesn' t remember he did it the next day.
So my mom and I are considering installing a porn filter in the computer. Pretty much we need a software which has some logic and block all the porn sites. Google recommended some following: OpenDNS (it seems not a parental control, but a great many are talking about it. Why?), Porn Blocker. But I have no idea which one to use (and the way). Any of you has experience with such kind of? Or some other nice advice without using software? Thank you!
Oh, we use Win 7 home pre and Win XP