|I don't see how using your computer at work would affect your devices at home.|
I have had comcast for years and and three house moves. Never had to download software.. ever. Activation was done by the comcast tech when on site.
Sounds to me like its your local comcast connection that somehow got crosslinked to another account and what you are seeing is a broadcast from comcast.
Couple of suggestions:
check your browsers setting for default page. It should not be comcast.
connect a pc directly to the comcast modem. Can you still not browse the internet?
disconnect your modem. call comcast. They will either direct you to a local location to get a new modem or come out to test/replace. If you get a new modem you will need to call them again so they can provision it. With a new modem this issue should go away.
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