WiredBrain as dealer says a defrag will not provide a way of securely deleting files.
Even commercial wiping software is not 100% guareteed to remove the data as MIT tests have showen it can still be recovered even after heavy software wiping and even degaussing.
The NSA did a test, they wiped at DoD standard multiple times, then sat a magnet on it for a couple of days, degaussed it, took it apart and smashed it with a hammer, they were still able to recover 80% of the original data!
However with wiping up to Guttmann 32 pass standards its very hard to recover any data without the use of a very expencive and time consuming process by which they read the data of the platters with special equiptment, i think they do analysis of the indentations left on the platter or something.
If you want to take advantage of secure wipe software i suggest you check out this page:
http://www.staff.uiuc.edu/~ehowes/soft10.htm
My personel recommedation would be BCWipe.
You may find this guys method interesting:
http://w1.270.telia.com/~u27007970/ghetto.htm
Anyway, simply no defrag does not remove deleted data! And as you can see commerical software is not the end of the story