Thinking of trading in your computer for a newer model? Be careful - you could be giving away the family silver.

Look at your computer. A bit dusty, perhaps. Maybe not as fast as you would like. And - unless it's bright blue and has a handle on the top - probably beige and terminally uncool.

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But be careful - it's just possible that what today you think is merely useful could one day keep you in your old age.

If you've got one of those early 1980s computers which played such an influential part in many people's younger lives, you could already be sitting on something of surprising value.

At least one fashionable British design firm is making the most of this popularity. Visitors to Deepend in the City of London are treated to a display of past computer glories, including the Sinclair ZX81 and Spectrum, three different models of the Commodore PET, a BBC Micro, an Acorn Electron, and an Apple II.

Takes you back

Design director David Streek said: "All of our clients, bar none, when they come into our office are fascinated by what they see. There's always something that triggers them into saying: 'I used to spend hours on that thing!'

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"It's very endearing to look at the old computers, with their controls and joysticks, they are really interesting. Nowadays computers pretty much look all the same, and I think that's why there's such an interest in the iMac."

He adds: "It drags something up from your childhood, to have a wood finish on a computer, like the Trinitron. It seems ridiculous now to make it like that, but at the same time it's really sweet."

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He has harnessed what started as a personal hobby into something that could be very lucrative - and no doubt a whole lot more interesting to his firm's clients than many a piece of modern art.

"I'm interested really from having them myself at primary school and the early stages of secondary school, when my friends and I all had computers," he added. "It was a rivalry thing." While he had a succession of Ataris, other friends were into Commodores, others still into Sinclairs.

"One of the best things about the UK computer market was you had actual computers, and that encouraged you to have half-hearted attempts at programming, whereas someone from the States would just have had a console probably, like the Playstation or Nintendo 64 are today.

"That's why so much gaming talent has come from the UK - there are lots and lots of romantic stories about the little English programmer who have become massive publishers."

Games driving demand

It's true that the demand for old computers is linked to the popularity of the early games. Graham Howden, who runs a computer fanzine Retro Classix, said lots of people were buying up old machines to run the old games - as opposed to using emulators on modern PCs which recreate the old games.

The Sinclair ZX81

"Hardened fans will want to own and play the games on all the machines the games were made for," he said. This means having a collection of old machines.

But there is a demand for the machines for their own sake.

"Lots of people are beginning to think of them like they were the fibre optic executive toys, and will sit there with them plodding away," said Graham Howden.

From running his fanzine, Graham was recruited by the Computer Exchange games chain for its nostalgia shop Retro, just off Tottenham Court Road in London. Interest in the old machines is now keen.

The Apple II

"If you came across a Sinclair ZX80 in your loft , that would be quite valuable. It was the white one that was produced in 1980, and even if it was a bit scruffy and wasn't working very well it would probably be worth £100." If it was in its box in its original condition, however, you could get up to £500 for it. A ZX81, which was much more common, would still fetch £60 if it was in its box.

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David Streek searches car boot sales for obscure old computers that the unwary are prepared to dispose of for a few quid. He is, however, phlegmatic about whether his collection is worth much.

"They are worth a hell of a lot to me, whether they are worth anything to anyone else I'm not sure. Although I can well imagine how comparably early train sets or teddybears have become valuable."

But how about the beige box on your desk? Is there any prospect of it one day becoming something more valuable?

Graham Howden seems to think so. "It undoubtedly will," he says. But you might have to keep it your attic for the next couple of decades before it finds its way into any exhibition. Whatever you do, just don't forget its box.

Pictures of old computers: Kevan Heydon