You may

erase a file

and you think it's gone, but it really isn't. You may

format the hard drive

and believe that everything is gone, but again it's really not. All that's really happened is you've flagged the space on your hard drive where the file(s) used to reside as over-writable. If you want to be really sure you have to

completely wipe a disk clean

before you give it away. Here's how to do just that.