All the boothelper files, which are no longer available directly from the Cisco CCO site can now be obtained from the Ciscon non-CCO site! This link will take you to all the relevant files and also contains additional information regarding password recovery on a PIX520.
The link shows how to download a working image from TFTP server to your PIX or ASA. You need to have a TFTP server to download the image into the PIX or ASA.
