Simple deleting a file doesn't help much because nothing is
deleted. The disk space is beeing made available again to
the system, but the actual bytes on the hard disk drive weren't
changed. The file system has to consume some time writing your
data to the disk, but it doesn't have to consume time 'unlinking'
it. It simply makes it available to the next best calling proccess.

A standard 'undelete' program can search through your hard drive's
free space and restore your files. Writing new data on a hard drive
minimizes the 'old data' to the point where your file system will
ignore it. But forensic soft/hardware used against you will be able
to read through.

Ground Zero secure delete is the solution !

With the support of this application, it is possible to
successfully defeat recovery tools such as EnCase to delete
a file in a secure way.

Advanced deletion shemes like the DOD 5220.22M Standard or Peter
Gutmann's algorithm ensure maximum security for your private data.
It is also possible to select your own custom deletion methode
where you can specify your own amount of overwrites.