A computer system includes a chassis and a fan module attached to the chassis. The fan module includes a rotor, a side panel coaxial with the rotor, and a cylindrical cover extending from the side panel for encasing the rotor. One side of the rotor is covered and protected by the side panel, while another side of the rotor is bare. The chassis includes an air vent for exposing the bare side of the rotor. An airflow-guiding part inwardly protrudes from an edge of the air vent and abuts the cylindrical cover of the fan module.