1. Line an 8 X 8-inch square pan with foil. Spray the foil lightly with nonstick cooking oil spray (or butter it lightly). Set the pan aside.
2. Pour the chocolate chips and peanut butter into a medium-size microwave-safe bowl. Microwave, uncovered on high, 2 minutes. Remove the bowl from the microwave, and stir well. Continue to microwave until the chocolate chips are completely melted, about 1 minute more.
3. Remove the bowl from the microwave, and add the raisins. Stir until they are covered in the chocolate mixture. Add the marshmallows, and stir gently only until they are covered with chocolate.
4. Pour the mixture into the prepared pan, and spread the fudge evenly in the pan.
5. Refrigerate at least 2 hours, until firm to the touch. Cut the fudge into 1-inch squares, and refrigerate in an airtight container for up to 3 weeks.