The most valuable part of your computer system was not supplied by Intel or Microsoft. You created it. It's your data.

If you were to lose your email, address book and financial records, it could take hundreds of hours to reconstruct or recover them. Worse, it might be impossible. If you were to lose your business records or the only copy of a blockbuster, the cost could be enormous. And it could happen easily, especially if you use a handheld or notebook computer.

You can lose data through hardware and software failures, and through human actions, such as deleting files by accident. You can lose everything if your computer is damaged, lost or stolen, which means portables are very vulnerable - just ask the Ministry of Defence.

The solution is a "back-up": a security copy on removable storage such as CD, tape or floppy disk.

Obviously the copy must be kept separate from your PC, in case of theft. If you run a business, back-ups must be kept "off-site", preferably in a fireproof safe. If the volume of data is not too large, you can upload it and store it on the net at XDrive, Bitlocker, MyDataHaven and similar sites.

By now, at least 90% of you are feeling guilty. Everybody is in favour of backing up data but few people do it, or do it often enough, including me.

But I am not saying you should take a full four-hour back-up of your hard drive twice a day. You only need to back up data you can't afford to lose. And you don't need to back it up again unless it changes.

Remember, you can always reinstall the operating system, and many new computers come with a bootable "recovery" CD that will return it to the factory state. You can also reinstall applications programs and games, though it may take time. What you cannot easily recover is personal data, unless you have backed it up.

Hardware

For home users, the main choice is between Iomega Zip disks that store 100 or 250 megabytes, and CDs that store 650MB or more. Although I use a Zip drive for temporary storage, I prefer CD-R (recordable) discs for permanent back-ups because they are cheap. You can buy 50 CDs for £16, so 1p buys 20 megs.

Ideally, the back-up drive should be built in to the computer, but you must remember to remove and file back-up disks somewhere safe. If you have more than one stand-alone computer, you could also consider buying a portable drive that plugs in via a USB serial port or parallel printer port.

Software

Basic back-up software is included in Windows - see Start|Programs|Accessories|System Tools, if it is installed. Better programs are often supplied free with back-up hardware, such as Iomega's 1-Step Backup, or can be bought separately. You can use back-up software to capture everything on your PC, and after that, take "incremental back-ups". In other words, back up only the files that have changed. The software will know which files to back up. It will also compress the back-up file (so it takes up less room) and split it over a number of disks or tapes.

However, it may not be quite as easy to restore the back up after a disaster, or retrieve a particular file in a hurry. It is, therefore, useful to keep plain (uncompressed) copies of important files on floppy or CD. If you can't get your computer up and running, you can still read these files on a different machine.

Another program worth considering is Second Copy which will keep a back-up copy of designated files, updating them as you work. Iomega's QuikSync works in a similar way.

Which files?

You can find which files have changed recently by searching for them by date. In Windows, for example, open Windows Explorer, select My Computer, and press F3. Select the Date tab in the dialogue box, and click the appropriate radio button. This routine lets you search for all files created, modified or accessed between two dates, or within a specified number of months or days.

Ideally, you should keep all your important data in one directory such as My Documents in Windows, or My Data on a different drive or a Mac. That will make it easier to back up. You can create subdirectories, such as My Mail, and then tell your programs to use these directories by default.

With Outlook Express, for example, you can change the directory by going to Tools|Options, clicking on the Maintenance tab, and pressing the button marked Store Folder. In Microsoft Word, go to Tools|Options and click the tab for File Locations. Choose the Documents entry, click the Modify button, and navigate to the directory you want.

You may also want to back up some files that you cannot move, such as your internet bookmarks. In Windows, copy the whole directory structure under C:\Windows\Favorites, though you can also export Internet Explorer bookmarks as a single file. Select File|Import and Export and a Wizard will take you through the procedure. With Netscape Navigator, keep a back-up of the Bookmarks.htm file.

Do it now, before it is too late!