OS X :: Networking Via Airport Extreme Or Belkin Share Wireless Router?

Since Saturday I've been having trouble connecting to the internet. For the last 18 months we've been connected to my Airport Extreme Base Station (2003) which in turn is wired to a Motorola SB5101 cable modem. Every time I had called the ISP (Comcast), they've been able to get it going while I was on the line by restarting the modem (which I tried before calling). Last night nothing would work no matter how many times I rebooted the modem and after many trial and errors, we were able to get connected to the internet by hooking my MBP directly to the ethernet cable from the modem and taking the Airport out of the equation. The service rep decided my router was likely at fault and I wasn't inclined to argue as it is an old router and things do break over time.

However, I ran out and bought a brand new Belkin Share wireless router and now neither that nor the Airport can connect to the internet even when the lights on the modem say all is well. Unfortunately, the Belkin came with it's own software package for router management and setup, which I'm not too keen on. It kept saying it couldn't connect to the modem or that the internet connection to the modem was lost. In tonight's efforts, I'm always able to plug my MBP right into the modem and get a connection, but as soon as I move that cable to the Airport or the new Belkin, my internet is gone. The Airport connection in the Network section of System Preferences always comes up as connected, but the Belkin glows dark red instead of green and the accompanying software says no connection to the internet can be found.

Does this sound more like the fault of the modem, the ISP, or me, the user? Any help would be much appreciated. Although I am able to connect now, my roommate's computer doesn't even have an ethernet port.