In reality, you aren't really deleting files. You are merely letting
Windows change the attribute of the file making it overwriteable.
It disappears from the file list because the file name is no longer
available on the directory list. So it looks like it's gone. It's better
to shred the file and then just overwrite it with other information
on the hard disk.
Programs like HardWipe, or Erase will shred the program and then
change the name of the file to make it less recognizable to another
user. Then you can defragment the drive and recover some space
where the file was on the drive. They call this defragmenting the
free space on the drive.
Here's an article from someone describing what it really means
when you delete opposed to shredding files. It also lists 4 free
shredder programs for you to makeuseof .com
http://www.makeuseof.com/tag/free-file-shredders-to-make-deleted-files-unrecoverable/