The country's largest provider of high-speed Internet to homes started testing a service this week in Denver in which Comcast sends customers a pop-up message in their Web browsers if their computers seem to have been co-opted by a botnet. One botnet can have tens of thousands or even millions of PCs.

The message points to a Comcast site with tips for cleaning infected computers. It reads: "Comcast has detected that there may be a virus on your computer(s). For information on how to clean your computer(s), please visit the Comcast Anti-Virus Center."

Comcast said users can close the warning banners if they wish, but they cannot opt out of receiving them. A reminder will return every seven days while a computer appears to be infected.

Comcast says its program contains an important secondary confirmation that the message is from the company and not a scammer: Comcast will send an e-mail to the customer's primary e-mail account.

However, Phil Lin, marketing director at network security firm FireEye Inc., said hackers could mimic Comcast's pop-up banner or the confirmation ads. And unsuspecting customers wouldn't know they should expect to see a confirmation from Comcast in the first place.