Abstract: A computer system and a method for playing encrypted multimedia data are proposed. The computer system has a first operating system that heavily consumes system resources and a second operating system that slightly consumes the system resources. Via a basic input/output system (BIOS), the computer system is booted with a second operating system. The second operating system uses a multimedia-receiving unit to receive remote encrypted multimedia data and store the data into a storage unit. The second operating system uses a decrypting unit to access at least one kind of the encrypted multimedia data stored in the storage unit and the register information corresponding to the encrypted multimedia data from a first operating system for decryption. After the decrypting operation is completed, a playback unit is used to access the multimedia data that has been decrypted for perform a playback operation.