download NIS or Anti virus but use disk product key?

My family acquired a 3 user version of Norton Antivirus and have installed it successfully on 2 computers.  However the 3rd computer is a laptop without a hard disk drive and they do not have an external hard disk drive.  Can they download the same prodcut from the Norton site and use the product key/serial number from the disk version to register the download?  if so can you breifly explain how?

Thanks Angie