Comcast, the nation’s largest residential Internet service provider, this week began rolling out an initiative to contact customers whose PCs appear to be infected with malicious software, by notifying these users via Web browser alerts. The Philadelphia-based cable Internet company has already been alerting bot-infected customers via phone for the past year, but a pilot program in Denver that began Thursday will inform affected users with a so-called “service notice,” a semi-transparent banner that overlays a portion of whatever page is being displayed in the customer’s Web browser. Customers can then either move or close the alert, or click “Go to Anti-Virus Center,” for recommended next-steps, which may include downloading and running the McAfee anti-virus tools the company offers for free, or purchasing a cleanup package and allowing a Comcast technician to attempt to remotely diagnose and fix the problem. Jay Opperman, senior director of security and privacy at Comcast,