Many people believe that simply by deleting a file that it is gone forever. On top of that many others believe that uninstalling and reinstalling your operating system the files have been permanently deleted from the hard drive, but neither of these methods actually wipe the hard drive completely, information stays on the drive and can be recovered in some way at some point in the future.
The only actual way to wipe a hard drive is to replace that information with lots of other information, like recording over a tape with something else. The previous information will be gone forever, totally wiped.
There are pieces of software that can do this for you. The process itself is can be natural in the sense that over a 5 year period, the information from 5 years ago could be wiped through the fact the computer has been used a lot over that time and has replaced that particular information, but this is never guaranteed.
The software I am going to suggest is through my own personal experience and does work very well. On top of that it is easy to use and will do the job properly first time.
You can find more information and purchase the software here at White Canyon
You can also find a whole heap of information on exactly how the software works and how it can work for you.