Sometimes the Internet Providers also add on antivirus program or a security suite to their users. Do you by chance have one of these in your email program through Comcast? The reason I ask is that having more than one AV could present a conflict on your machine. Usually the AV offered by the Internet Providers is an option, but is often pushed upon the user and not a very effective AV at that. I also do not use Comcast, so other than this, I am out of ideas as well. If you are unsure about if they added on a security software, you can call their customer support to find out.