What is the maximum distance from the cable modem to a PC ?

When connecting through Ethernet, the max distance is 100m (300 ft). If the connection is through USB, distance is much more limited. The max USB cable length is 5m, so even with a few USB hubs you won't get very far.

There are several techniques availible to extend the distance of Ethernet beyond the 100m. One is adding repeaters, another is adding a media converter at each end to convert from UTP to fiber and back.