硬盘的工作原理是你删除了文件，其实文件还在硬盘上，只是操作系统标注这块地方可用，以后有新数据可以再往那快地方写罢了。
要真正删除数据只有一个方法：反复往硬盘写无意义的数据，写上千遍，这样，就很难找到开始时的数据了。
举个例子，如果一张白纸比喻成硬盘。那么你在用圆珠笔写字。字是不能擦的。你要写新的字，直接写在以前字上面。新的字自然会盖住下面的字，但有些下面的字还是能认出来的。如果你往纸上写1000层无意义的字，你开始写的什么字理论上应该被覆盖得完全看不出来了。
这就是我理解的彻底删除数据软件的工作方式。
好了，说这款自由软件。是工作在windows上的。因为同事用windows。
下载： http://heidi.ie/eraser/download.php
介绍：
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. Works with Windows 95, 98, ME, NT, 2000, XP, Vista, Windows 2003 Server and DOS.
Eraser is Free software and its source code is released under GNU General Public License.The patterns used for overwriting are based on Peter Gutmann's paper "Secure Deletion of Data from Magnetic and Solid-State Memory" and they are selected to effectively remove magnetic remnants from the hard drive.
Other methods include the one defined in the National Industrial Security Program Operating Manual of the US Department of Defence and overwriting with pseudorandom data. You can also define your own overwriting methods.
Erase files and folders securely.