reply to GaryK
Re: PC/Activity light flashing on modem The PC Activity light has nothing to do with other users. It's sole purpose is to indicate the status of the Ethernet (or USB) connection between the cable modem and the customer equipment (in this case, your router). You can test this for yourself by turning the power off to the router. There is no indication at all of any traffic outside of your premises.
On solid = problem with modem or (more likely) your equipment or software configuration.
Off = The cable from the modem to your equipment is disconnected, defective, or equipment powered off or broken.
Flashing = Data is being transferred.
Just download the user's manual for the Surfboard modem; the first few pages describe the functions of each of the status LEDs on the front of the modem.
Even with my PCs turned off, my Linksys router generates internet traffic (albeit little). So what you're seeing is normal and desirable. Everything's fine!