When you put a file in your “recycle bin” or “trash can” on your computer, it is not really gone – not even after you empty the recycle bin. The information can be recovered with freely available software. Granted, over time the file becomes harder and harder to recover, but it can be recovered.
You can permanently erase a file though. Free programs such as Eraser allow you do erase single files, folders, or even nuke the whole drive. As a matter of fact, wiping the whole drive is the only way to insure that the files can’t be recovered.
In the past, people have touted the fact that erasing over a file once was not secure enough. As a matter of fact, most of these programs will do up to a 35-pass wipe of the file or drive. It looks like you don’t need to be that paranoid though. In reality a single pass will do the trick.
even firms specialising in data recovery, openly admit that if a hard disk is overwritten with zeros just once, all of its data is irretrievably lost.
http://www.h-online.com/news/Secure-deletion-a-single-overwrite-will-do-it–/112432
You should also remember that encrypting your whole drive is a much better solution for data protection. You only need to wipe the drive if you think your encryption password has been compromised – or just change the password.