In a large non-stick frypan saute the chicken breasts. Add salt and pepper to taste. Add the mushrooms, red capsicums and yellow capsicums and cook until vegetables are tender.
Cook the spaghetti according to packet directions. Drain and set aside.
In a large saucepan melt the butter or margarine and blend in flour. Gradually stir in the chicken stock and milk. Cook over medium low heat, stirring constantly, until the sauce begins to thicken. Add garlic salt and ground black pepper to taste. Blend in the Swiss and Parmesan cheeses and continue heating, stirring constantly, until the cheeses melt.
Stir in the chicken/vegetable mixture and heat thoroughly. Toss with the cooked pasta and top with grated Parmesan cheese, if desired.