There are people who can't wait to get their hands on your old PC, writes Vanessa Spedding

Growing pressure on businesses and personal users to keep up with the latest in computer technology, to keep upgrading, means that somewhere there must be millions of redundant computers. Where are they, and what happens to them?

Well, the Midland Bank gives some away to deserving causes, so does the accountants KPMG. Beneficiaries to date include Guide Dogs for the Blind, the Scottish Homeless Youth Project and Hitchin Sunday Football League. But such measures are a last resort, used only for machines with negligible resale value. Those that can be sold or part-exchanged for new equipment disappear into an underworld of second-hand dealers.

At some point they reappear in second-hand shops, at auctions or in the small ads. If you want a second-hand computer, have a look at Micro Computer Mart or PC Mart - though Yellow Pages may also point you in the right direction.

But the route a computer takes from desktop to desktop can be a tortuous one, during which it may undergo any number of humiliations, which can bring risks for the buyer. An apparently modern machine may be full of cannibalised parts, which may or may not work properly. So make sure you get a guarantee.

The computer recycling industry is full of all sorts, ranging from large companies to "bedroom brokers", working from home to put together functioning machines from second-hand bits. Midland Equipment Brokers of Bromsgrove buys mixed bags of unwanted computer equipment, mainly from companies in the South-east with a high technology turnover. The broker's job is usually simple - check the computer for faults, reformat the hard disk and sell it on to the public via retail outlets or mail order.

Demand, says Midland Equipment's Carl Fontenari, is huge: so much so that he is having to import from Germany to meet it. This results in fierce competition between brokers - sources of equipment are heavily guarded secrets.

A small dealer in Wales confirms the strong demand, and says he is interested in any computer of less than 10 years old. He is coy about where he buys from, but one source would certainly be computer auctions which are on the increase and doing a roaring trade. A typical auction will accept goods from companies and individuals alike and sell to a mixed bag of the public and trade customers.

Some computers will be cannibalised, and the parts will then enter a different sales channel. Specialist reprocessing companies accept consignments of unwanted circuitboards - second-hand or new rejects - and pick out the "populated" ones for chips and components, which are tested, repackaged and sold on as "second-user" parts. Some resellers may be less than rigorous about the components they resell. Where parts are scarce - as SIMMs, computer memory chips, are - factory failures may be re-marked and sold as good. Likewise chips from used computers are sometimes passed off as new.

The computer giants have to be very careful that substandard computers carrying their brand names - or their branded parts - are not pushed on to the open market. According to Carole Redman, IBM's recycling manager, the group tries hard to use unwanted computers and working parts elsewhere in the company: some are taken by the maintenance engineers for outside repairs to clients' machines. If re-use is impossible, machines are scrapped by appointed merchants following strict procedures. For IBM, proof that computer shells and parts bearing their identity have been fully "disabled" (smashed up) is required for each consignment.

But even a scrapped part can have value: circuitboards contain precious metals which are reclaimed and reused elsewhere. Currently, the only parts to end up in landfill are the monitors, as lead in the screen prevents recycling.

A small electronic scrap merchant in Hereford, John Hunt Metals, makes a tidy living from obsolete printed circuitboards. Many are sold back to specialist chip recycling firms and end up in other products such as toys or burglar alarms, as well as in second-hand computers.

Peter Foweraker, who runs the company, describes electronic scrap as the "scrap of the future", with volumes increasing daily. Bits that cannot be resold whole may yield gold, silver, platinum, steel, aluminium and copper. He prefers older machines as new technologies employ such metals in ever decreasing quantities. Dated computer equipment imported from eastern Europe has gold "trowelled on", he says; where last year's circuitboards might be worth 30p a kilogramme, equivalent boards made 20 years ago fetch more like £3.50 a kilogramme.

So if you're thinking of throwing your old 286 into the skip, stop. It's worth something to somebody. Old computers don't die, they just get reprocessed.