Files are never "deleted" from your computer.
When you delete a file, all you are doing is telling your computer that you are no longer using that piece of data and that the operating system can over write the data if it needs to.
The only REAL way of deleting a file from your computer is to use a software package that literally goes over all of the spaces on the disk that the operating system isn't using, and over writes it with "junk".
There are a few free software packages out there that can do this. Google "Hard Drive Wipe" and you should be able to find a free program you can use to go and write over all of your "blank" data.
Keep in mind though, even doing this isn't 100% fool proof. These wiping programs use an algorithim (formula) to write over the data. It is still possible to use sophiscitaced "unwiping" software to go back through and unwrite what it over wrote.
Because of this, if you really want to get rid of your data, run the wiping several times, and do so using different programs. Even then, it is still possible to recover "erased" and "wiped" data.
The only sure fire way to make sure the files are completly deleted, is to physically destroy the disk the files are stored on.