Basically if your Format was NOT unconditional it can be retrieved, how data is deleted is basically tagging the items on your HDD with a unique key code or whatsoever to render that specific file's used space as being writable to, so basically if you formated the HDD, and installed something over it, you might of loosed some of your files, because the installation may have written to a location on the HDD which had files on it and was marked for writing over. There's many software utilities to actually visibly see and restore these tagged data files which are rendered writable.