I dont think anyone make a program that will do this on its own AS you delete things through normal computer useage. What you could use to wipe the Free Space (actual free space, or files that you have deleted and are registered as free space) would be
CCleaner or
Eraser.
With CCleaner you can wipe the entire free space with an overwrite standard or 1time, 3times, or more. Very similar with Eraser, but with Eraser you have an option to delete single or multiple files, folders with Eraser's overwrite deletion standard of 1time, 3times, or more. Eraser can also be used to wipe free space, clear the recycle bin, or delete the entire hard drive (so be careful!).
You do realize that by doing this constant secure overwrite of your free space and of everything you delete, if you need to recover a file you deleted accidentally, its nearly impossible!