Get a clean(new) sh404sef.class.php file then edit it with Notepad++, not with Wordpad

If the problem persists download-upload the sh404sef.class.php in BINARY mode of your FTP program.
For CuteFTP go to Tools=>Global Options=>Transfer=>>Transfer Method

You can also check this:

http://docs.joomla.org/Cannot_modify_header_information_-_headers_already_sent