Four passengers were injured during the emergency evacuation following an APU fire. The crew reported the APU fire bell while taxiing. The captain elected to evacuate, but did not indicate the nature of the problem to either the cabin attendants or passengers. Several passengers used the overwing exits on the same side as the fire and were injured jumping off the wing. One said that the wing flaps were not lowered during the evacuation. The APU had been placarded inoperative prior to the flight. Crew advised in writing. The cockpit voice recorder tape was erased in the cockpit after the accident, so the flight crew's activities were not able to be reviewed. Post accident examination of the APU revealed that the fire started because of an attempted start of the defective unit. Prior to the flight, NWA maintenance attempted to make the APU inoperable from the cockpit by opening circuit breakers. The wrong circuit breaker was pulled, leaving the APU operable.