Heat oil in a Dutch oven over medium-high heat. Add onion and garlic; sauté 4 minutes. Add curry powder, and sauté 1 minute. Add squash and next 3 ingredients. Bring to a boil; reduce heat, and simmer, uncovered, 30 minutes.
Place half of squash mixture in a blender; process until smooth. Repeat procedure with remaining squash mixture. Return to pan. Stir in sour cream and milk; cook over medium heat until thoroughly heated.