Cant connect to the internet

I have an imac g3 running mac os x 10.4.11, and i have a modem from comcast, a motorola sufboard sb5120 cable modem and when i connect the router to my PC via USB it works and the same thing with ethernet, but when connect either or to my mac it won't connect to the internet,  i have all the settings on the mac right i have tried connecting via ethernet directly with a wired router and a wireless router via hard wire, and i have tried with a 24 port ethernet switch and still nothing, i can see the files on my pc if i connect to the pc using all of the ways I tried above connecting to the internet, but i still can't connect to the internet, please help.