Coincidentally, I just saw a similar system at instructables.com (was poking around waiting for the forum to re-open) using Staples Easy buttons.
http://www.instructables.com/id/Quiz-O-Tron-3000-Arduino-quiz-contestant-lockout-/
That one doesn't have the PC software to display scores tho.
How do you sample when buttons are pressed? Is it possible to have 2 pressed at the same time?