There's eBay, of course. There are plenty of second-hand computers on sale, but of course, the usual caveats apply about checking the provenance of what you're being sold, and making sure the seller is legitimate. The seller's feedback rating can provide some of that, but even then you may be without a warranty.

One way around that is a refurbished computer - when people send in theirs for repairs under the warranty, if it can't fix them the company will send out a new one, then refurbish the old one and sell it on at a discount. A number of manufacturers sell their own refurbished kit on in this way (Dell refurbished store, Apple refurbished store).

Some of them sell their refurbished stock on to other companies who sell it for them, such as Euro PC, MCS and Morgan Computers (Morgan has offices in London, Manchester and Derby, while Euro PC has a shop in Glasgow). It's possible to pick up excellent bargains at these shops if you know what you're looking for, and not just for computers - they sell monitors, projectors, music players and cameras, among other things.

Finally, some of the bigger shops run their own clearance sales - both Comet and Currys run auctions of 'clearance' products. Those tend to be the same deal - refurbished products that have come in for warranty repairs, or that have been used by customers and then returned. Of course, both of those retailers sell lots of products other than computers, so there are plenty of electrical and electronics bargains to be had.