Saute' onion, celery and bell pepper in butter until soft. Add powdered milk, cornstarch, chicken boullon, pepper, 6 cups reserved water (add more if necessary), Rotel Tomatoes, and cubed Velveeta Cheese. Cook until cheese is melted. Add spaghetti. Mix well and put into two large 9x13 inch sprayed pans and top with shredded cheddar cheese. Bake 45 minutes to 1 hour.