I'll try to help. But first, some questions:
I guess your cable modem is also your wireless router/DHCP server/etc? Or if you're not sure what capabilities it has, is the modem the only network device, other than PCs? Do other PCs work when plugged into the same ports/cables?
We'll try to troubleshoot the wired connection first. So, boot up your machine, then plug in the ethernet cable. Do the LEDs next to the network jack light up? If they do, then run "dmesg | tail" in a terminal to show the last couple messages in the system log. You should see something like this: "[229858.805222] b44: eth0: Link is up at 100 Mbps, full duplex.". Next, run "ifconfig" to list network info. Does the "eth0" device have a "inet" address listed?