When you delete a file, the operating system does not really remove the file from the disk; it only removes the reference of the file from the file system table. The file remains on the disk until another file is created over it, and even after that, it might be possible to recover data by studying the magnetic fields on the disk platter surface.
Before the file is overwritten, anyone can easily retrieve it with a disk maintenance or an undelete utility.
Eraser is an advanced security tool for Windows which allows you to completely remove sensitive data from your hard drive by overwriting it several times with carefully selected patterns.
http://eraser.heidi.ie/download.php
