The pilot advised that during the engine run prior to take-off the windscreen fogged up both internally and externally. She shut down the engine and wiped both surfaces free of moisture. There was light fog present in the area. During the subsequent takeoff run the windscreen again fogged over and she lost visual reference. The pilot closed the throttle and rejected the takeoff. However the aircraft diverged off the runway and impacted trees before coming to rest substantially damaged. The four occupants were able to evacuate unaided and were not injured.