Do these popups say "Messenger Service" in the title bar?
If so, then you are using an older version or (un-updated) of Windows (pre XP SP2). These type of popups exploit the Messenger Service in your Windows OS, which after XP SP2 is disabled by default.
A popup blocker "thingy" will be of no help in this case.
You do NOT have spyware installed.
You have 3 choices to end the problem.
1. You can disable and stop Messenger Service by typing "services msc." (without the ""s) into your Run Box and then disabling and stopping Messenger Service.
OR
2. You can use the free utility called Shoot The Messenger to automatically disable this unnecessary service.
http://www.grc.com/stm/shootthemessenger.htm
OR (Best Solution)
3. Update or upgrade your operating system to at least Windows XP Service Pack 2 (SP2).
Please note that Windows Messenger Service is not the same as any Instant Messenger program that you may have on your computer. Disabling it will not cause you to lose any IM functionality that you may have.
BTW, Comcast only provides the internet to you, NO ISP will maintain your computer, that is your responsibility!
Good luck