Sorry but as nobody else is reporting such an issue then it has to be assumed that the issue is something within your setup.
From what you say you have no issue downloading or installing CCleaner, the issue only happens when you try to run it.
What AntiVirus and/or AntiMalware are you using?
I suggest that you check your Firewalls etc.
Plus - despite what you say about Comcast re-boot your router.
Have you tried running CCleaner in Windows Safe Mode?
You haven’s said - Was CCleaner working OK for you and this issue started suddenly?
Or is this a first time install and you have not had CCleaner running on the machine proviously?