In a large greased bowl, combine the popcorn, cereal and M&M's; set aside. In a heavy saucepan, combine the corn syrup, sugar and butter. Cook and stir over low heat until sugar is dissolved. Add peanut butter; stir until blended.
Pour over popcorn mixture; toss gently to coat. Spread into a greased 15x10x1-in. pan. Cool before cutting.