The problem wasn't to do with the seller's description of the bargain buy, but what the listing hadn't mentioned.

Logging on to the computer, he found at least a million details for bank customers from around the country. As well as account information, it also included addresses and telephone numbers – even customer signatures.

Computer security expert Edward Wilding said: "The banks had out-sourced the archiving and disposal of materials and something went wrong," he says.

"Things do go wrong and you can put in the best security system imaginable but something, somewhere is going to go wrong, particularly with companies of this size."

However, Wilding – who is the director of computer forensics company Data Genetics International and the author of two books on computer security – says he was surprised by the scale of the faux-pas.

"What did ring alarm bells was the extent of it," he says. "It's been reported there's a million records on that computer.

"Why are people walking around with millions of client confidential records on a computer? That's alarming."

One-off cases like these may make national headlines, but what is less likely to make the front page is when people sell on their old personal computers and forget they are giving away their own details for free.

So what can you do to ensure your data doesn't end up in the hands of crooks?

Nigel Smart, professor of cryptology at the University of Bristol, says that most of the information you'd like to remain private will be found in one place on your PC.

"Most of the stuff is the data stored in the browser, like bank account details and passwords," he says.

"Because we can't be bothered to remember them all, they are stored in there."

Smart says the easy way to delete your protective passwords is to wipe your hard disk and re-format it.

"In Windows there's something that does it for you, you click on a button which says about reformatting the hard disk and it asks, 'do you really, really want to?' You've just got to be very brave and say 'yes'."

He says that will wipe the operating system so the average criminal would not be able to recover the data.