It's unlikely that the malicious link was actually on Comcast's page. While it's possible that someone snuck malicious content onto their website, it's far, far more likely that it was injected after it hit your network.
There is some malware that targets commodity home routers and will inject similar Flash Player downloads into pages. If you haven't updated your router firmware recently (or since you bought it), you might want to do that. Those attacks tend to reside only in memory, so power-cycling the router should clear the malware temporarily. Update it as soon as it comes back up, and you should be protected from that class of attack.
The other possibility is that you already have malware on your machine, and it's trying to trick you into installing additional software for more/better control over the system. Running a good virus scanner or anti-malware package would be a good first step.