Stir together popcorn, cereal, raisins and nuts in large microwaveable bowl; set aside. Combine butter, brown sugar and corn syrup in small saucepan. Heat until boiling; cook for 3 minutes, stirring occasionally. Pour over popcorn mixture, stirring to coat evenly. Microwave 3-4 minutes, stirring and scraping bowl after each minute. Spread onto greased cookie sheet; cool. Break into pieces and store in airtight container.