Boil pasta in a medium saucepan as directed on the package. Drain, then return to pan. Add butter, milk, and macaroni cheese seasoning from the box (And/or Velveeta, to desired cheesiness!). Set aside.
While pasta is boiling, brown your beef in a separate skillet over medium-high heat until no longer pink; drain. Return to pan.
Add water and taco seasoning mix to the beef. Bring to a boil, stirring occasionally. Add Rotel tomatoes and chiles, then reduce heat to simmer. Simmer for 5 minutes.
In a large serving dish, combine the beef mixture and macaroni and cheese. Serve immediately.