In windows when you delete a file or folder its link is only delete,but the file remains in your hard disk it can be recovered by using software like recuva . Most of the antivirus has the option called file shredder to completely delete a file and the shredder file cannot be recovered. Securely file shredder does the same ,its a portable and a tiny application.To completely delete a file or folder just drag and drop those thing to this application,it shows the warning that it cannot be recovered press yes that's all the files and folder are completely deleted and it cannot be recovered .You can also shredder the files from recycle-bin by using a option shown below in the image. While selling your old pc or laptop this helps a lot to erase your personal files and folder so it cannot be recovered by the buyer.