Why is Fallout 3 so laggy on my PC?

I just got Fallout 3(not New Vegas) for my PC and it's laggy as hell! The cutscenes are choppy and the audio is horrible and incomprehensible. I checked on the "can you run it?" thing before I even thought about buying it, and my computer surped the minimum requirements. When I first started it, it checks your computer and automatically sets it to the best settings for your computer.