There are some programs available that can "wipe"
the free space on your computer. Generally what these programs
do is they would write 1's to the free space, then 0's and then
1's and 0's together.
One program that wipes the entire drive and prepares it for
sale/disposal is Autoclave. (http://staff.washington.edu/jdlarios/autoclave/)
Autoclave is a free program that has 5 levels of security, but
it erases your entire drive so that it is unrecoverable.
There is an interesting paper on the topic from sans.org that
explains the problem in great detail at http://www.sans.org/rr/paper.php?id=631.
There is also a comparison of file wipers for windows computers
at http://www.fortunecity.com/skyscraper/true/882/Comparison_Shredders.htm.
If you have/use a macintosh there are fewer programs, but they
are still effective. One that I have used comes with Norton
Utilities, but there are others like the program 'Burn.' (ftp://ftp.thenextwave.com/pub/mac/burn/burn25.hqx)
We consulted with Leonard Miller, who appeared in the "