So just to reiterate, this is a "transparent message during browsing," meaning it appears as something like a pop-up (but not) that is semi-transparent. It's not a normal browser popup in a popup dialog window (no frame) that can be closed.
Despite all the helpful comments, this is NOT due to software accidentally installed on a computer. You can walk into a friend's house with a laptop or tablet, get their wireless network password, connect to their network, open a normal non-SSL HTTP connection to a web site, and wah-lah you start getting this annoying semi-transparent overlay if it's active. And if your browser app isn't windowed (e.g., handheld device), you can't move it.
Comcast claims they only do this once a week if a device on your home network is detected as having a bot, but I've seen it more than that. Your only recourse is to fid out WHICH device on your home network is causing this to happen, and figure out how to clean up that one device. If you have lots networked devices on your home network, this can take a long time. Try leaving certain devices off for multi-day periods, and see if it goes away, plus obviously doing virus scans on all your home computers, etc., not just the one you happen to get this message on.
Remember, Comcast is an ISP. They completely control your Internet connection, including all layers of the IP protocol stack and the physical layer transport. Their routers and networking equipment have full understanding and filter control over the HTTP transport protocol used by web apps (including browsers), TCP, IP and data link layer packets below that.
What they are apparently doing as an ISP ostensibly "protecting their network" is using something comparable to packet injection to insert additional HTML content defining this semi-transparent message box into the otherwise normal content being received from a web site (doesn't matter which) HTTP socket connection. It is not something accidentally installed on your computer, which is obvious given that it shows up on ALL computers, tablets, cell phones, etc., connected to your home network.
Can I opt out of receiving future Service Notices?
No. The Constant Guard Service Notices are necessary to protect the security of all our high-speed Internet customers. Under the XFINITY Internet Acceptable Use Policy, you have the responsibility to secure any equipment that connects to the Internet via our service from external threats such as viruses, spam, bot nets, and other methods of intrusion. Comcast is sending this Service Notice to assist in that effort. As a result, this is similar to any other service notification we might send. You can close the Service Notice without any further action. We may notify you again in the future if your security is at risk.