Preheat oven to 400°F. Pierce potatoes several times with a fork. Bake about 1 hour, or until crisp outside and cooked through. Transfer to a baking sheet and cool 5 minutes. Cut off top third of potatoes. Scoop out potato from bottoms into a bowl, leaving 1/4 inch thick shell. Scoop potato from tops and add to same bowl. Discard tops. Mash potato with half the sour cream, half the horseradish and margarine. Season with salt and pepper to taste. Spoon into potato shells, dividing equally. Place potatoes in baking pan and bake about 20 minutes, or until heated through and golden brown. Mix remaining sour cream and horseradish in a bowl. Spoon sour cream topping on each and sprinkle with chives.
