Concern here about the affect comcast's actions could have
http://www.sophos.com/blogs/chetw/g/2010/10/04/comcast-provide-opportunities-fake-av/
That really makes a joke of the endless time spent on forums everywhere trying to educate folk into watching out for messages like that appearing on your screen, and how to ignore them and avoid falling into their traps.
Someone in Comcast needs a transplant between the ears.
If they want to protect their subscribers I think they should do it via email instead of having a suspicious JavaScript pop-up -- even if it were legit. Moreover I don't think it's completely necessary if people have good updated daily anti-malware software installed and regularly run system scans.