I’m selling my old computer; how can I be sure that passwords and personal information aren’t hiding somewhere on it?

While the people who write computer programs strive to give us an “intuitive end-user experience”, with well-organised menus and enticing buttons to push, most people – myself included – have no idea what’s happening behind the scenes. Sure, we usually remember where we’ve saved any documents we’ve created. But temporary files are often created automatically by the computer and stored in unusual locations; this is often for our own benefit in the case of a crash or power-loss, after which clever auto-recovery features kick in. In addition, preferences and passwords might be found in a “Preferences” or “Settings” folder – but, equally, they might not. Essentially, the contents of our hard disks are something of a mystery. Even when we delete files that we no longer need, they’re not really deleted; the computer merely marks that area of the disk as “ready for overwriting”. As a result, data that you thought had long-gone may well be still there, and files that you’d deleted by accident can often be recovered using inexpensive utilities.

This is great for the absent-minded and error-prone person who has accidentally trashed their 80,000-word autobiography. But it also means that second-hand computers can be mines of fascinating data, full-to-bursting with old files that can be brought back to life. After selling your computer on eBay, you might be boxing up what you think is a machine containing no personal information – but unless you know how to permanently delete it all, that’s probably not the case.

Simply reformatting a hard disk and reinstalling the system software does make a computer look “as-new”, and will usually dispel curious new owners from taking a peek at the previous contents. But the Royal Canadian Mounted Police, apparently, recommend a triple overwrite with random data in order to be completely safe – and, should you wish to emulate their stringent, exacting standards, there are ways of doing so, without paying a penny. Steve Bryant writes: “On Macs, when you reinstall OS X, you can either choose a quick format, or a “zero data” option, which overwrites your entire disk with zeros.” PC users, however, will need to get hold of some special software to accomplish the same thing. “There are a few paid-for solutions,” writes Jon Mackereth, “but there’s a free one called Eraser (www.heidi.ie/eraser) which is available for most version of Windows, and has won an award or two.” Be warned: zeroing data is a lengthy process, but it can be worthwhile – particularly if you’re working high up in the world of espionage. Whatever you do, though, do make sure you’ve backed up all that precious data before you start.