In a large heavy bottomed stock pot, heat up enough water to cook your fettuccine and cook it according to package instructions.
While the fettuccine is cooking, dice up your vegetables.
After you have drained the fettuccine, set it aside in the strainer, and return the stock pot to the medium low heat. Melt the butter, and add the garlic, cooking until it is just fragrant. Then add the bell peppers and onion. Cook for two minutes, stirring regularly.
Add the shrimp and cook it with the vegetables, stirring regularly, until the shrimp is pink, about five minutes.
Stir in the fajita seasoning, and coat the shrimp and vegetables with it. Turn the heat low and add the pasta back to the pot, and then stir in the alfredo sauce, until everything is coated and creamy. Serve immediately.