I am glad my kids have my old PC, but as they have gotten more savvy, I needed a good porn/violence filter that was easy enough to use and configure, and not so intrusive as to make me disable it. I did not want it hard for them to be able to build an animated Neo Pet, nor difficult for me to read Yahoo! News or Google some interesting factoid.
I tried Internet Explorer's built-in Content Advisor, which either a) blocks all Internet content except for explicitly allowed sites, or b) allows a parent to lean over the kids' shoulders and type in a master password, that will then allow the kids to visit the site (with options to 'always allow' or 'just allow this time'). The problem is that the Content Advisor is turned on, across all Windows users. Since we have multiple Windows XP users, on the one PC, this became a hassle, since even my wife and I use that PC on occasion, and were thus forced to input the Admin password every time we browsed to somewhere new.
So, I disabled Content Advisor, and searched for something else. K9 Web Protection (http://www.k9webprotection.com/) seems to a good solution. It works from a database, via the WEB, of nasty sites, and blocks them. The Admin can get around it, if one likes, but for the very most part, problems do not arise. It is easily customizable, via an Admin screen accessible via one's WEB browser (though it exists locally on the PC). You can get this free product at the site mentioned above, free for home users (but charged for corporate users).