BOSTON — Operating systems are a lot like movie directors: They control everything but stay out of the limelight. An operating system (OS) is the key software that directs a computer's hardware and runs the rest of its software. If you have a desktop computer, your OS is probably Microsoft Windows or Apple's Macintosh OS. Corporations have several other choices to run their large computer systems, including Windows NT (a network version of Windows) and Linux. The best OS is fast, runs lots of programs, and rarely crashes.