As the resident computer geek in an office full of accountants, my boss recently asked me how she could reasonably keep her teenage son from using the family computer to "access inappropriate sites." I of course responded "Give up now. There's nothing in this world that can keep a determined teenager from acquiring porn." Sadly, she was dissatisfied with this answer. I mentioned that there was in fact software available for this purpose, but that all of it was trivially easy to bypass for a clever young mind. (Beyond: watch him constantly or just deal with it like the adult you intend to raise him to be.)
I really can't think of another answer. She could password protect the BIOS to prevent booting a different OS, but that's easily defeated with a screwdriver at most. The only solutions I can think of involve upstream firewalls/proxies/etc to which I gleefully redirected her to her ISPs tech support number.
As much as I disagree with her reasoning — and ignoring the obvious "go to a friend's house" loophole — is there really any other way (on a home budget) to netnanny a household computer? (she does sign my paychecks...)