Often when you try to delete a file from disk, it isn't really deleted. Instead the information about it's existance(eg. what sectors it is using), but not the file itself. Because it, have thousands of programs, that undelete your data from disk.
If you want to really delete data you need to replace
it by something else. The program
shred -u <filename>,
included in the GNU/Linux and GNU/Hurd(coreutils package)
securely delete your files.
But if you have deleted your files in the wrong way then
you can fill your hard disc several times with random data.
You can copy your secret files to a encrypted file system
see Quick start. Then fill your hard disk with random
dd if=/dev/urandom of=/mnt/random_data). You
can delete /mnt/random_data after it.