DOS
DOS (disk operating system) was an operating system created by Tim Paterson and made famous after Microsoft purchased it in 1980. One of the early drawbacks of DOS was its inability to deal with large amounts of memory. It also had a very simple, command line interface, which meant that users had to type in commands in order to execute programs and manipulate files.

Windows
Development of Windows (previously known as the Interface Manager) began in 1981, and it was released in 1983. Using DOS as its basis, it generated a GUI (graphical user interface) that used the same metaphor as the Mac OS, with files and folders sitting on a desktop. This type of interface became known as WIMP - Windows, Icons, Menus and Pointers. There have been numerous updates and upgrades of Windows, and one of the main drawbacks for users is the lack of compatibility between versions. A program that may run on Windows 98 may not run on later versions, with the possible exception of software developed by Microsoft themselves. Operating systems continue to grow in size, demanding ever increasing amounts of hard drive storage space, more RAM (random access memory) and faster processors. For smaller computers, such as PDAs (Personal Digital Assistants) and handheld computers, a pared down version known as Windows CE has been created, with the look and feel of the familiar Windows 95 and NT.