Cook spaghetti according to package directions. Meanwhile, in a bowl, combine the cornstarch, water and broth until smooth; set aside.
In a large skillet, sauté garlic and cayenne in oil until tender. Stir broth mixture and add to the pan. Bring to a boil; cook and stir for 2 minutes or until thickened.
Reduce heat; add shrimp, lemon juice and zest, and parsley. Cook for 2-4 minutes or until heated through. Drain spaghetti and place in a large bowl. Toss with the sauce to coat.