You can also use the BCWipe utility.
BCWipe utility provides 4 ways to shred files:
-Delete with wiping
-Wipe free disk space
-Swap file wiping
-Files slacks wiping
And it can wipe directory entries.
There is an option to wipe according the U.S. DoD seven pass extended character rotation wiping (DoD 5200.28-STD).
After wiping your files with the option mentioned above, you can not recovere the files by any means!
You can download this program from this site, it's freeware http://www.jetico.com