So what on earth does a computer operating system do?

In a computer, the operating system performs the same function as the staff in a hotel. It takes care behind the scenes of everything that you, the user (or guest) need, to turn an unwieldy object - the inert computer, or empty hotel - into a personalised environment. The operating system allocates space for the programs (the computer equivalent of guests) who want to use the computer. When all goes according to plan, the operating system, like good staff, is just an invisible presence.

In technical terms, the operating system is itself a computer program which runs other programs and looks after the hardware of the machine, such as its central processor, hard disc drive, floppy disk drive, keyboard, screen monitor, CD-Rom, speakers and external connections to networks or telephone lines. If a user wants to run a word-processing program, the operating system finds that program on the hard disc and loads it into the computer's memory. It then runs it, while picking up the input from the keyboard and displaying it on the monitor, and saving any data to the disc drives.

The concept of the Windows operating system, and of the rival Apple's Macintosh computers, is that rather than typing commands to make the system perform a task, you move a pointer (the handheld "mouse") across the screen and click a button on it.

Microsoft dominates the market for operating systems of personal computers: Windows already runs on 100 million PCs worldwide, compared with the 20 million using Apple Macintoshes. And Windows 95 marks a notable advance - though not a revolution - over the current version. "It's like putting new soles, heels and polish on an old pair of shoes," says Lawrence Magid, a US consultant. "It makes the PC look and feel better without disturbing the user's comfort."

With Windows 95, the screen will be less cluttered and easier to use. It should run programs more quickly, and be able to run a number of different programs at once. It will be able to sense when a new piece of hardware, such as a printer or speakers, is attached to the computer. And it will have a link to Microsoft's own online computer service, which will have connections to the Internet, the worldwide network with 30 million users.

Apple Macintosh users say all these are hardly innovations, as they have been available on their systems for years; some refer to Microsoft's new product as "Macintosh 89". But this ignores the market dominance of Windows. A common joke in computer circles is: "How many Microsoft programmers does it take to change a light bulb?" To which the answer is, "None - they just redefine the industry standard as darkness."