Preheat oven to 200. Heat sugar, margarine, corn syrup and salt in a small saucepan, stirring occasionally, until bubbly around edges. Continue cooking over medium heat for 5 minutes more. Remove pan from heat. Stir in baking soda, blending until mixture becomes foamy. Add peanuts, if desired. Divide popcorn and spread on 2 cookie sheets. Pour liquid mixture over popcorn, stirring until corn is well coated. Bake 1 hour, stirring every 15 minutes. Cool and serve. Gift Idea: Put the finished product in decorative tins and give as holiday gifts!