"Who do you want to be, IBM or Sony?" That's the question the marketing guru Regis McKenna is said to have asked Apple. But every supplier has to decide if it is in the computer business or the consumer electronics business.

They are not the same.

Computers are flexible and functional devices that are judged by their performance, and some are much better than others. Their appearance is either secondary or irrelevant. If you're spending $20m (£12.5m) on a server, you'd have to be stupid to worry about whether the doors are red or black. It's going to be locked in a darkened room, and hardly anyone is going to see it, unless it goes wrong.

Consumer electronics devices are, by contrast, very often judged on their appearance or, more accurately, on the image developed and supported by advertising and marketing. You can even sell products that have badly designed user interfaces and offer mediocre performance, as the average clock-radio demonstrates. As far as most consumers are concerned, one TV set or video recorder is basically the same as any other TV set or video recorder. In consumer electronics, design fashions change like hemlines, and for much the same reasons.

Computers are designed as "open systems" to enable them to be adapted and upgraded. (In this case, "open" just means you can get at the insides, though it can mean more.) Often this means adding memory and hard drives, but it can mean changing the processor, graphics card, modem, or even the motherboard. It can mean replacing a CD-rom drive with a CD-R rewriter and then a DVD writer.

Consumer electronics devices are usually, by contrast, sealed boxes. The parts are fixed so that they cannot be upgraded, and they may not even be worth repairing: either you can't get the parts or the labour cost is too high for them to be worth fitting. You throw it away and get a new model with the latest black/silver/translucent plastic design.

Computers are being upgraded on a continuous basis. In fact, modularity is an essential part of the design, because all the parts are developed separately, and not all the different technologies develop at the same rate. Computer manufacturers exploit this to sell faster and more powerful systems every year, but at much the same price.

Consumer electronics manufacturers, by contrast, want to sell you exactly the same device for years, but either for less money, or a bigger profit margin. Designs may get tweaked, but the main driver is cost reduction through component integration. Once a games console has been launched, for example, that's it. The manufacturer's challenge is to reduce the complex original to a couple of integrated chips that do the same job for less money.

All this applies at the extremes, where the product is either a multi-million-pound computer server or an ultra-cheap plastic handheld game. But many products, while clearly in one camp or the other, have elements of both - or can be treated as if they did. There are, for example, hi-fi geeks who open "sealed box" products and change or modify some of the parts.

The more interesting question is: where do personal computers fit in?

There is no doubt about the historical answer: personal computers are upgradeable platforms. There have been dozens of computers that have been sold successfully as consumer electronics products, such as the Sinclair Spectrum, Commodore 64 and Amstrad PCW, but they did not enjoy the longevity of upgradeable platforms.

The most famous example is the original Apple Macintosh, which flopped when it was launched as a sealed box product with a built-in monitor, in 1984. It was only by copying the PC industry's three-piece approach - system box, monitor, keyboard - with open expansion slots, which it did with the Mac II, that Apple was able to rescue the platform. But as the successful iMac and failed Cube designs show, the desire to sell computers as consumer electronics products has not gone away.

Apple co-founder Steve Jobs has always taken Sony as his model, and in 1998, told Fortune magazine: "The whole strategy for Apple now is, if you will, to be the Sony of the computer business." However, Jobs clearly understands that Sony has, in fact, two different businesses: one supplying consumer electronics products and one supplying professional markets. Hence there are different ways of packaging Macintosh technology for consumer and professional use. And although the iMac has enjoyed only a limited success, it has been far more successful than its "classic" ancestor.

There is another indicator of what may be changing times: the arrival of the "desktop replacement" portable computer. These are big, heavy beasts with limited battery life, and they are clearly unsuitable for portable use. Buyers also sacrifice most of the expandability and future-proofing of desktop platforms: they are basically sealed box systems that dare not speak their name. However, they appeal to people who don't want to devote the house room to a desktop computer, preferring one they can fold up and put away.

There are also reasons why a shift in the market makes sense. First, personal computers are now so cheap and so powerful that all the things most people need can be packaged in a single box. Second, the buying demographic has changed. PCs are no longer the preserve of a small proportion of geeks, they are bought by millions of ordinary consumers who don't need expandability, and would be too scared to take the lid off if they did.

But I wouldn't take any bets on the market changing dramatically. PC prices depend largely on economies of scale that are reinforced by volume sales to businesses. Given the shape of the standard building blocks, it becomes increasingly expensive to vary the shape of the result.