We've got the computer in the living room. The occasional porn add or google spam did pop up. We didn't mind since it was obviously accidental on the kids'part but it bothered the kids so they approached me for a solution. Firefox, addblock and foxfiltre help a lot. Registering with google and youtube (dummy account!) and setting the preferences to safe did the rest. They can surf the net without supervision, they know I do random checks of the browser's history and everyone's happy.