Cook pasta according to pkg. directions.
Meanwhile, in a microwave-safe bowl, combine cheese and milk. Cover and microwave on high 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 ham, peas and cheese sauce. Transfer to a 2 qt. baking dish. Cover and bake at 350 for 30-35 minutes or until bubbly.
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