Boil chicken in a large pot (5 to 6 quart). Remove chicken and debone. Using the same pot, cook the spaghetti in the chicken broth (if you have a lot of broth, you can pour some off before cooking the chicken and reserve for later use if needed). Once the spaghetti has cooked, turn heat to low. Do not drain the noodles. The broth is the base for your spaghetti.
Cook the onion and bell pepper in the margarine until tender; add to the spaghetti and broth, then add the Rotel and Velveeta. Cook over low heat until cheese melts, stirring as needed to prevent sticking. Add the chicken. If the spaghetti mixture appears too thick, add some of the reserved broth.
Pour the spaghetti mixture into a large baking dish, at least a 9x13 (may have extra, as this makes a lot). Bake at 350 until the spaghetti is bubbly around the edges.
Makes a large amount of spaghetti. Excess or leftovers can be frozen for later.