Do you have a cable modem and a router, or just a cable modem? Is your PC plugged directly into the cable modem or router? Are you sure the plug is in the right connector on your PC? Many PCs have both 100 megabit and gigabit ethernet ports today, and they may not both be enabled.
Try this: power down the computer, the power down the cable modem and router if you have one. Now power up the cable modem and let it run through its self test (i.e. wait a minute or so until the lights on the front settle down). If you have a router power that up next and wait until it settles down. Now power up the computer.
If that doesn't work, with everything running, go to Start Menu -> Run and type cmd.
In the text box that appears, type ipconfig /all and copy the resulting output to your next message here. We'll see if we can't figure it out.