Given that data is invisible, it's not hard to let your PC become overrun with it. A good clear-out freshens up a PC no end, and if you're disposing of a computer there's an even better reason to clean it up.

Whether giving an old computer to charity, selling it online or passing it on to a friend, you'll want to make sure that contacts, web browsing history and your personal information isn't included in the donation. Read on to find out how to ensure that personal data is properly deleted from a hard disk.

Indestructible data
Disposing of data from a hard disk is not as simple as putting it in the Recycle Bin; the computer will erase its record of the file but the information will remain on the disk somewhere. Web browsers also store lots of important information that you might not want to share and it is advisable to delete information regularly.

Destroying data is difficult and even damaged disks can yield secrets when examined by forensics experts. There's no need to worry about electronic criminals stealing data from PCs that have been thrown away, as the process is complicated and expensive. No thief, even with the skills and equipment, would bother.

But properly erasing files and the history of websites that have been viewed on a PC is a courtesy to new owners and can improve the computer's performance.

There are two types of data that need deleting: internet files and saved data. With the former, regular scheduled computer maintenance should be carried out, as this improves your computer's efficiency. Saved data deletion needs different techniques and special tools, but you can download a free utility from our website, which we will get to shortly.

This should be done before giving away a computer or to destroy sensitive documents. First we'll discuss some of the issues to consider and then explain how to erase data beyond any practical use.

To delete data safely from a disk, it helps to understand how it gets there in the first place. The heart of a hard-disk is the platter, a set of highly polished plates, split into smaller compartments called sectors, that spin at speeds approaching 200mph.

Data is stored on the disc magnetically, so you don't lose everything when you shut down the power. A read/write head, which looks like the arm of a record player, moves over the disk and writes information onto each platter. Every time data is written to the disk, a record is kept of the part of the platter on which it has been stored.

When you put a file in the Recycle Bin, the file itself isn't erased; Windows merely discards its record of where the data was stored. As you continue to use the disk, the computer will eventually overwrite the original data you stored with new information.

But think of your hard-disk as a large library. All the books are numbered and their location stored on a card file. If the card file was destroyed, a search through every volume would need to be carried out to find a particular book. But they would all be where you had left them - on the shelves.