mdturner has inadvertently given you links to the normal Symanatec version of Norton 360 whereas you apparently have the COMCAST version and need a different set of files -- I don't even know that there is a single download installation file for COMCAST or only the Download Manager version.
mdturner's route is a good alternative to the Download Manager for the standard Norton versions and using the single file download sometimes avoids installation errors.
Try the route that SendOfJive gives to a document specific to the problem from the sound of it.