Software managing the computer hardware. Operating systems vary in their make-up because they are organized in different ways, and designing a new Operating System is a major undertaking. Because an Operating System is complex, it has to be designed one piece at a time. Moreover, each piece needs to be a well-defined section of the systems, with well defined inputs. For PCs, the most popular current operating system software is the Microsoft Windows family, but experts project that Linux will replace Windows on at least one-fifth of all computer systems by 2010.