Why? Because, preposterous though it sounds, “deleting” computer files means no such thing. That command merely tells the computer that the space the information once occupied on the disk is available to overwrite, and anyone with a modicum of technical nous can easily recover it. Even software packages that promise to zap all your personal data almost always fail to do so, one expert insists. Worried? Well, you should be.

Amir Tofangsazan found that out the hard way. He sold his laptop on the auction site eBay, but the buyer was none too amused when the computer arrived broken. Tofangsazan, allegedly, failed to refund his cash, so the new owner took sadistic revenge: he removed the hard disk from the broken laptop, scanned it with another computer and found all Tofangsazan’s juiciest data, including details of his personal life and salacious photos, which he duly published on a website. Tofangsazan denies that much of the content is his, but the damage is done to the reputation of the 18-year-old, who hopes to become a barrister.

Will reformatting a computer’s hard drive prevent it from later divulging your secrets? Not necessarily — even free utilities, downloaded from the internet, can dig up old files from a reformatted machine. What about software packages that claim to erase all your sensitive data? Dr Andrew Blyth, from the school of computing at the University of Glamorgan, insists they are hit and miss. Earlier this year, his team randomly bought 200 second-hand hard disks in online auctions. They found that nearly one-fifth of them still contained sensitive information such as company files and bank- account details.

Blyth, who is also a police forensic-data expert, has tested numerous commercial and free software-erasers and reckons that “very few” of them work. He says of some such packages: “If you consider a hard disk as a book, all they did was to tear out the index and leave the pages intact. If you plugged in a forensic tool, you could recover the data.” Though the likelihood of someone acquiring your computer and running forensic techniques on it is slim, Blyth insists that there is “a genuine risk”.

If you want to wipe your old hard disk to British government standards of security, as he suggests, he says that the only product he would recommend is Blancco’s Data Cleaner+ (£16.95 from www.tinyurl.com/e65oo). The package writes over the contents of your hard disk with random data up to seven times. However, wiping a computer’s primary hard disk means it will no longer start as normal. The new owner will need an operating-system CD-Rom or a recovery disc to restore the computer to working order — which may hamper your ability to sell it on.

Be cautious before handing your machine over to recycling companies. Though most claim to wipe your data, Dr Blyth says that many use inadequate erasing software. He suggests asking whether the company runs “a freebie tool or a commercial product that has been accredited by an independent organisation”. After all, if my computer’s going to end up in a country such as Nigeria, with its appalling record for identity theft, I’d want to be damned sure my bank records couldn’t be retrieved.

In the end, experts advise, the only way to be absolutely certain that your hard disk won’t betray your secrets once you’ve handed the machine on is to put it through a grinder. A waste, don’t you think? So, here’s a challenge for Bill Gates: instead of wasting time on designing flashy see-through menus in Vista, the next version of Windows, ensure that pressing the delete button does what it says. Until that is the case, anything you drop into your computer’s Recycle Bin really could be recycled.

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