Combine first 6 ingredients in a large bowl; stir in squash.
Melt the margarine in a large nonstick skillet coated with cooking spray over medium-high heat. Add the hash brown potatoes and sliced leek, and sauté for 7 minutes or until lightly browned. Add the egg mixture to skillet. Cover, reduce heat to low, and cook for 10 minutes or until the center is almost set. Uncover, and sprinkle with cheese. Preheat broiler. Wrap handle of skillet with foil, and broil frittata for 5 minutes or until the cheese melts.