Melt margarine in a large saucepan over medium heat. Add onion; sauté 4 minutes. Add celery and carrot; sauté 6 minutes. Add potato and next 5 ingredients (potato through broth). Bring to a boil, reduce heat, and simmer 40 minutes.
Place chestnut mixture in a blender; process until smooth. Return to pan; stir in milk. Cook until thoroughly heated. Ladle soup into individual bowls; top with croutons, and sprinkle with paprika.