Halve leeks lengthwise and cut enough crosswise into 1/4-inch pieces to measure 2 cups. In a bowl of cold water wash leeks well and lift from water into a sieve to drain. Peel potato and cut enough into 1/4-inch pieces to measure 1 cup. In a 1 1/2-quart saucepan cook leeks and potato in butter with curry powder over moderate heat, stirring, 5 minutes. Stir in 2 cups water and simmer, uncovered, 20 minutes.
While soup is cooking, cut spinach into thin strips. In a blender purée soup until completely smooth (use caution when blending hot liquids) and return to pan. Add milk and salt and pepper to taste and bring to a simmer. Remove pan from heat and stir in spinach.
