A computer system includes a display device, a video output interface, and a control module. The control module includes a central processing unit, a multimedia processing unit, and an output switch circuit. When the computer system is powered off and a multimedia mode is entered, the multimedia processing unit is capable of processing signals from a multimedia source of a storage medium. The output switch circuit includes inputs X1 and X2 respectively coupled to the central processing unit and the multimedia processing unit, and outputs Y1 and Y2 respectively coupled to the display device and the video output interface. The input X1 or X2 of the output switch circuit is selected to receive the signals from the central processing unit or from the multimedia processing unit, and the output Y1 or Y2 is selected to output the signals to the display device or to the video output interface.