Well firstly they could have the options lower, you could have some crap running, drivers could have set AA, they could have tweaked windows, they could have overclocked something, they could have different drivers, your PC could be overheating (Niko missed one! :P), you could have custom textures.
If you want help, could you be less vague? If i walked into a garage and said "yeah, i have a.. um... nice car and my friends um... car... is faster, but its um... not as nice. Tell me whats wrong!" you wouldn't get far, so we need details!