This sounds like a known scam: the Reveton virus (a.k.a. Moneypak virus) used in conjunction with Citadel malware. It installs itself onto your computer from a hacked (or malicious) website, and then accuses you of copyright violations, underage porn use, and/or other crimes, demanding that you follow instructions to pay a ransom to unlock your computer.
It wouldn't be surprising for the bad guys to target Comcast customers using the Comcast name and links.
If your ISP were giving you an actual copyright warning, they wouldn't f*** up your computer and browser; they'd use email, snailmail, and/or the phone.
The IC3 [Internet Crime Complaint Center] suggests the following if you become a victim of the Reveton virus:
- Do not pay any money or provide any personal information.
- Contact a computer professional to remove Reveton and Citadel from your computer.
- Be aware that even if you are able to unfreeze your computer on your own, the malware may still operate in the background. Certain types of malware have been known to capture personal information such as user names, passwords, and credit card numbers through embedded keystroke logging programs.
- File a complaint and look for updates about the Reveton virus on the IC3 website.