A computer system having an operating environment configured for enabling boot up in a relatively short time is disclosed. A hard disk is utilized to store a boot file. A run process list registers a process to be run during boot up. When the computer system is being booted up, if the run process list registers a process to load the boot file, a process control unit does not control a process management unit, so that the process is run as usual. If the process is not registered, the process management unit suspends the process. When the suspended process is called by another process later, the process control unit makes the process resumed on demand.