The airplane's engine starter was inoperative so the pilot decided to hand-start the engine with the propeller to get it started. He had his wife, who was not a pilot, sit in the airplane and hold the toe-brakes with her feet. The parking brake was not set. When the engine started, the airplane began to roll forward, and the pilot's wife was unable to stop it from moving. The wife stated she was scared of the propeller striking her husband so she turned the airplane away from him. The airplane traveled off the pavement, through a ditch, across a soybean field, and collided with a parked airplane resulting in substantial damage to the left wing. The parked airplane was also damaged. The pilot said the starter was the only mechanical problem with the airplane prior to the accident. He said it was a poor decision to hand-start the engine with an inoperative starter and he should have secured the airplane until it could be inspected by a mechanic.