Yes, it is true. When you delete data it isn't actually removed from your computer. At first, the information is moved to the trash or recycle bin. When you empty the bin or the bin clears itself, the computer changes the name slightly so the operating system can't see it. Over time the data will be eventually overwritten by newer information.

It isn't unknown for old data to sit on a computer for years. This can be embarrassing or worse when throwing or giving away a computer. A couple of years ago one British buyer of a second hand laptop found his new computer contained Paul Macartney's financial records. Before getting rid of an old computer you should delete any important or personal data and make sure it is permanently deleted.

The most simple (and least certain) way is to empty the recycle bin and then run defrag, you can make this more effective by starting in Safe Mode and deleting the swap file (search for *.swp) before running defrag. This way will overwrite most deleted data.

A more certain way is to repartition and reformat the hard drive. Surprisingly, this alone doesn't guarantee old data is gone forever. But once you have reinstalled your operating system and applications then you can be reasonably sure it is all gone.

The most certain way of destroying old data is to use a "shredder" program. This overwrites the space a number of times and makes sure the data is unrecoverable. These programs are included as part of utility packages or can be downloaded free from sites such as Tucows or C-Net, just do a search for "shredder".

Don't assume you have nothing to hide because you don't get up to anything untoward on your computer. If you are regular computer user you will have passwords, unsent correspondence,draft documents, financial statements and details that could at the very least embarrass you.

When deleting data, make sure that data has been copied, backed-up or isn't useful before permanently erasing it. Once it has gone it is gone forever. Lost data is the only irreplaceable thing on your computer.