As I mentioned in this SuperUser question, I'm trying to prepare a PC for educational use by a child, and am trying to find free parental control software to keep installed and ready (their parent might choose to go for some paid application later, but for my part I wish to help them by having a free program ready to use).
From some Googling, it seems that: for Windows, Windows Live Family Safety, Qustodio and K9 seem to be some good options, while for Linux (Ubuntu) "Web Content Control" seems to be the only current option (at least based on this Ubuntu Wiki page).
Since it's a decade-old Pentium 4 PC, the biggest constraint is the resource usage - the more lightweight the program, the better. Other than that, being able to manually add blacklist entries and make exceptions to existing block-lists (if any) would be a requirement. The software must not lump pornographic content and mere profanity and must allow selectively filtering out such categories independently.
Being able to block particular applications with particular schedules, having a simple UI with offline help available, generating easy-to-understand statistics from any monitoring done - these would be nice-to-haves.
If anyone here has experience and expertise in this area, please make a recommendation, whether for for Windows (XP) or Linux, or both.