Hi,
You require eraser software to wipe your hard disk, so that nobody will recover any data from your hard disk.
Formatting the hard disk will not erase the data inside the hard disk. It will remain in the hard disk. In order to remove permanently, you need to overwrite the hard disk with random data using software
Check the below link for the free utilities
http://geeksaresexy.blogspot.com/2006/08/disk-wiping-utilities-how-to-get-rid.html
http://www.whitecanyon.com/wipedrive-erase-hard-drive.php
D.Jeya Kumar(JK)
MVP
http://tamilpadamcomedy.blogspot.com