What I've seen for other Kiosk systems here is a
physical device with a coin-regulated timer that
is hooked up between keyboard/mouse and computer.
As I said in my first post: it depends on what way
you're going to run the thing.
Have a look on http://freshmeat.net
- search for
"Internet cafe" or Kiosk; you might be surprised
how many are about and freely available.