Preheat oven to 350ºF. Spray a 9-inch square baking dish with nonstick cooking spray.
Bring a large pot of water to a boil over high heat. Add spaghetti and cook according to the package directions for al dente. Drain and set aside.
Cut up chicken into small bite-size pieces and season with taco seasoning.
In a large skillet melt butter. Add cubed chicken, garlic, and onion powder. Cook over medium-high heat until chicken is no longer pink and no liquid is left in the skillet.
Stir in soups, Velveeta cheese, and Rotel Tomatoes. Reduce heat to low and cook until the cheese melts, stirring constantly.
Stir in spaghetti and pour into the prepared dish. Top with shredded cheddar cheese.
Bake for 25 to 30 minutes or until heated through.
