I am running my computer with Windows ME and Red Hat Linux 7.1. The cable modem works fine in windows, but it will not work in Linux.
Tell us what kind of cable modem you have.
I had the same problem with my leased 3Com "Shark fin" cable modem and every distro I tried. I also have a 3Com 900b NIC card and knew that wasn't the problem because it worked fine in *nix with my previous cable modem. The "send" light would light up but got no return response so I knew it was probably not my setup. I assumed it was something about Linux networking not recognizing the ISP's DHCP setup. I played around with different configurations, searched for answers through every forum I could find, but never had any luck and gave up in frustration for a while. I can't get rid of the modem because it's leased and my provider gives no support if I roll my own. So, like trickykid, I figured if I could find a router that had its own internal DHCP server maybe I could get Linux DHCP to recognize that and play nice with my modem...
I ran out and bought the Linksys Etherfast 4 port cable/dsl router for $79. It took roughly 2 minutes to install and configure... I powered everything up and the usually annoying, but this time heavenly beep tone sounded on my modem signifying I had made a successful connection. Yay! Works like a charm with Mandrake 8.1. As an added benefit, it acts as a firewall with NAT and allows for 253 computers to be networked onto the same IP with additional hubs. $79 isn't cheap but I couldn't find any help and I was tired of 'nix without net. They also make a 1 port cable router which is cheaper than the 4 port.
Hope this helps...