I'm going to need additional information about how it's not working. Most cable modems come with an external interface box that takes a crossover cable from a NIC in your server. If the cable modem itself is faulty, or you aren't using the crossover cable supplied by the vendor, you won't see the three green lights on the modem.
If the cable modem is working and the cable is installed correctly, focus attention on the NIC. Does the network interface show up in Networking and Dial-up Connections? Are the IP parameters configured correctly? Some cable modem providers stipulate that the machine must have a name they provide and they use this name as an authentication mechanism to get the DHCP options. Have you configured the name correctly? Use IPCONFIG to check the IP address, gateway, and DNS settings.
Are you able to ping the DNS server? If not, you have a stack configuration problem. If so, then you may be trying to route onto the Internet unsuccessfully. The simplest way to set up a server as an Internet gateway is to configure Internet Connection Sharing (ICS). Open Networking and Dial-up Connections and right-click the external interface (the one going to the cable modem.) Select the Sharing tab and check the "Enable Internet Connection Sharing..." option. The system will change the IP address of the internal interface to 192.168.0.1 and install a mini-DHCP service and a DNS forwarder. Restart other clients to get the new DHCP configurations and you should be off and running.
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