Psst! Wanna buy a used operating system?

Microsoft said that DOS was dead, but now they've changed their minds.

Microsoft, it has to be said, is not good at keeping its promises. Anybody with old copies of computer magazines can find fulsome promises from the corporation's executives that Windows NT Version 5 (since renamed Windows 2000) would be released by, oh, the end of 1998. Current forecasts are that it won't be available until 2000 - and there's still time for that date to slip.

Missing deadlines is one thing, but reversing strategy publicly announced by Bill Gates is another. Yet that is what Steve Ballmer, Microsoft's president, did last week, by announcing that in 2000 there will be a new version of Windows 98 - still based on the creaking MS-DOS operating system.

That followed a similar announcement, made last Tuesday, that users would have to pay for a CD-rom of bug fixes for Windows 98 - followed the next day by a "clarification" that, in fact, the bug fixes would be free (and available over the Web), but that the CD-rom would contain updated drivers and new software, and so cost money.

Faced with this, you may wonder whether Microsoft's left and right hands (and its two left feet) know what they are doing. But there is a common thread. Microsoft is so enormously profitable - with net profit margins of more than 30 per cent on sales (most companies manage 10 per cent) - that it needs a continuous stream of revenues from old products just to stay still. With Windows NT5 receding like the horizon, it needs something else: something old.

Why should MS-DOS form part of that? Because it is much easier to rejig the Windows 98 operating system than Windows NT. The former is intended for consumers, the latter for businesses that need something solid, and that will take Microsoft to task if it is not. The public holds no such leverage.

MS-DOS was Microsoft's first product; and even that was not its own. It paid $50,000 for the rights to Q-DOS (for "Quick and Dirty Operating System") written in six weeks around April 1979 by a programmer at Seattle Computer Products who was trying to clone Digital Research's eight-bit CP/M operating system for Intel chips.

The Windows 98 or Windows 95 that you see on your machine today may look flashy, but, in truth, it is communicating with the hard drive and processor by way of MS-DOS. Windows 98 was meant to be the last gasp of MS-DOS; after that the operating system would be rewritten wholesale, with a new "kernel" and interface to the chip. That's Windows NT.

Obviously, a consumers' version won't be ready next year. Hence Ballmer's announcement. Yet you have to wonder what, if anything, can possibly be squeezed on top of Windows now. "New features," promised a Windows product manager; which experienced users will interpret as new incompatibilities and bugs. Some people never learn, and, happily for Microsoft, many of them buy its products.

Case in point: the CD-rom (expected to cost $30) due in the autumn will "upgrade" Windows 98 with bug fixes, new USB drivers and home networking capabilities, plus Internet Explorer 5.0. You might not see anything worth spending money on there, but Microsoft is sure many people will. And that's a promise.