welll it really depends on the file system and the OS..in FAT when you delete a file, replaces the first character of the file name in FAT entry with a '?' and this is used to recognize the deleted file to restore it u need to supply the correct first word and replace it with the original character
Yep, that would be the flag to overwrite the disk space occupied by the file.

On another note, you can get shredder which will overwrite the disk area and make it *extremely* difficult to restore the file. Even then, you may only get bits and pieces of the original content.