Cut potatoes into 1/2 inch cubes. On a cutting board (use a separate cutting board to reduce the risk of contamination), cut the chicken breast into same size cubes. In a microwave safe dish, mix the potatoes, the chicken, tomatoes, broth and the taco seasoning. Cover and microwave on high power for 15 minutes, or until the potatoes and chicken are done. Let soup sit for 2 minutes.