Kill disk will wipe your whole drive.
None of the deleted files are using memory just disk space (in a way).
When you delete a file, windows just wipes the first letter off the filename and therefore can no longer find it, the space on the drive is then available to windows to write another fie over the top as required.
This erasing of the first letter is what allows recovery programs to retrieve deleted files.
You can get programs to securely delete the file by writing over it several times. However the space on the drive is still occupied but available to windows.