In a saucepan, add milk, powdered sugar and margarine over medium heat. Bring to a boil, stirring constantly. Stir constantly for six minutes after it starts to boil (if you have a candy thermometer bring the temp. up to 234 degrees). Turn the heat down to low and make sure it keeps bubbling (if it's not bubbling turn the heat up a bit). Stir in chocolate bits, peanut butter, and marshmallow. Stir until everything is dissolved and very smooth.
Pour in a 9 x 13 x 2 cake pan lined with foil. When it's cool enough, refrigerate for several hours, then dump fudge upside down on cutting board and peel off foil. Cut into squares. Keep fudge squares refrigerated.