In a skillet, cook beef over medium heat until no longer pink; drain. Add taco seasoning and prepare according to directions Add cream cheese; cover and simmer for 5-10 minutes or until melted, stirring occasionally.
Transfer to a bowl and refrigerate for 1 hour. Cook pasta according to directions.
Drain and toss with butter (a couple of teaspoons to keep the shells from sticking). Fill each shell with about 3 tablespoons of meat mixture.
Place 12 shells in a greased 9 square pan; cover and freeze for up to 3 months. To prepare unfrozen shells: spoon salsa into a greased 9 square pan.
Top with shells and taco sauce.
Cover and bake at 350 degrees for 30 minutes.
Uncover and sprinkle with cheese and chips; bake 15 minutes longer or until cheese is melted.
Serve with sour cream and onions.
To prepare frozen shells: Thaw for 24 hours and cook as above.