A computer system capable of playing multimedia files while in a power off mode or power-saving mode is provided. The computer system has a first operating system operated in a high power mode and a second operating system operated in a low power mode. The first operating system has a hard disk storing multimedia files and a central processing unit for running the first operating system. The second operating system has a non-volatile storage unit consuming substantially less power than the hard disk, for storing multimedia files; a multimedia playing unit for playing the multimedia files of the non-volatile storage unit; and a microcontroller having less computing performance than the CPU and consuming substantially less power than the CPU, for running the second operating system when the first operating system is in a power off mode or power-saving mode. A related method for operating the computer system is also provided.