In a large skillet, heat oil over medium-high heat; add stew beef and garlic and brown lightly.
Add chili powder, cumin, cayenne pepper, and salt and stir to coat beef.
Add tomatoes and water. Bring to a boil, then reduce heat, cover, and simmer 1-½ hours, stirring occasionally.
Add spaghetti sauce, then continue to cook, uncovered, 10 to 15 minutes, stirring occasionally, until heated through.
Meanwhile, in a large pot of boiling salted water, cook pasta to desired doneness; drain and place in a large bowl.
Spoon beef mixture over hot pasta and serve.