As you probably know, there is no such thing as deleting from disk. It's not like a cassette tape where you just wipe it, because if you did, you'd destroy the formatting.
Whilst I cannot guarantee it will do the job you want, if you want to keep the system files, I've just downloaded a freeware utility which does the job very well.
Its called "Eraser" and it purports to be just as secure as many professional (and expensive) programs.
It can overwrite individual files multiple times with a large cryptographically secure random number sequence before marking as deleted. This is (apparently) to "military" standards, and is the equivalent of shredding. It can also overwrite all your free space in a similar manner to deal with previously (and historically) deleted files.
The original developer is no longer supporting it, (due to other commitments I understand), but it has been taken over by someone else. Although they do ask for a $10 donation, it
is totally FREE, being licenced under the GNU public distribution licence.
Click here for ERASER
Good Luck,
Andy Woolford