Belkin Router won't allow two computer to connect?

My friend just bought a router last night after finding a hub wouldn't work to split an internet connection. It's a Belkin Wireless G Router (Model number: F5D7230-4). The easy install didn't like the modem we're using. We use Comcast cable and a Scientific-Atlanta, Inc. modem that Comcast sent. I accessed the router through it's IP address and changed the MAC address. I was able to get the computer I got the MAC address from (with ipconfig /all) to connect to the net afterward, but when I hooked another system to the router the internet light went off and stopped the internet feed completely. I tried just pulling the second computer's cable to see if the internet would work again, but it didn't. I also went and added the MAC address I got from the second system to a list of filters to allow both systems to work with the router (after connecting the second computer itself to the cable modem). I also enabled DHCP. Thanks in advance for you answers.