Sharp Microwave Oven. I bought a Sharp Model R-403 KKT, Serial Number 59333, microwave oven

Question

I bought a Sharp Model R-403 KKT, Serial Number 59333, microwave oven somewhere around April of 2007. About 2 months ago as I was warming a covered plate, I heard a loud cracking sound from the microwave oven. When I turned to look the covered part next to the light was on fire. I immediately unplugged the unit and stopped using it. Have there been any other reports of such things happening? Since I no longer have the original carton and receipt I cannot return it to Wal-Mart where I bought it. What are the possibilities that Sharp will replace this unit?