When you delete files in Windows,you are in effect turning them into free space by modifying them slightly,so that Widows "knows" it can freely overwrite them if that space is needed. Sadly,this slight alteration means that your local nosey nerd could possibly still find out quite a lot about your files even tho Windows now sees them as free space,as probably only the first letter of the filename is changed,say to an "x".
A file called "Offshore account"-deleted might show up as "xffshore account" on your hard drive-not exactly secure if Windows has not yet got around to using that space!
Now,a 35 pass overwrite surely will make all the deleted material virtually unrecoverable-but the CIA might manage it! I would for normal purposes,just do a few random passes,but I leave that to you.
Erasing free space means just that,but erasing the complete drive means you want to remove everything,Windows and all-not normally required,except if you are disposing of the computer-unless I misunderstand your question!