On January 20, 2001, at 1700 central standard time, a Cessna 172F, N7882U, experienced an in-flight fire of unknown origins during takeoff from runway 36, Greenwood- Leflore County Airport, Mississippi. The personal flight was operated under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane was destroyed. The commercial pilot received serious injuries. The airplane departed Greenwood-Leflore County Airport, in Greenwood, Mississippi, at 1700. According to the pilot, after departing Greenwood-Leflore County Airport on runway 36, he noticed smoke in the cabin. He reported, he turned around and saw flames behind him. He turned off the master switch, and turned back towards the airport. He tried to put out the flames but was unsuccessful. During the return to the airport, the airplane collided with power lines. Then the aircraft clipped a 6' high airport security fence. The pilot landed on the grass next to the runway, and exited the airplane. According to the air traffic controller, the airplane climbed to approximately 35 feet after take off from runway 36, and suddenly the pilot executed a left turn and returned to the airport. The controller observed smoke coming from the airplane as the pilot maneuvered the airplane for an emergency landing on the airport. The pilot made an emergency landing in the grass adjacent to runway 36. Examination of the maintenance records revealed that, no evidence of fire occurred prior to the modification. The airplane maintenance records revealed that a radio instrument rack, and other electrical equipment had been installed in the passenger compartment prior to the flight. The airplane's passenger compartment from the firewall to aft of the baggage compartment was totally consumed by the fire after landing. This flight was the first flight after the modification and the fire started in the area where the new equipment was installed. The fire ignition source has not determined.