Cook pasta according to package directions. Meanwhile, in a microwave-safe bowl, combine cheese and milk. Cover and microwave on high for 2 minutes; stir. Heat 1-2 minutes longer or until smooth, stirring twice. Stir in ketchup and horseradish until blended.
Drain pasta and place in a large bowl. Stir in the ham, peas and cheese sauce.
Transfer to a greased 2-qt. baking dish. Cover and bake at 350° for 30-35 minutes or until bubbly. Yield: 4 servings.