Issues running my pc and Xbox 360s through the same Modem-Router setup

[Digital Robot]

I just moved into my brother's apartment and I brought my D-Link Switch, my old-school SOHOware SFH105A Router, some ethernet cables and myXbox 360 with me. However when I tried to connect my xbox through my D-Link switch (via our Motorola sb5101 Modem, we have Insight cable) it wouldn't connect, there was an ip address failure. The pc worked fine. I disconnected the pc and restarted the modem and the switch and then the xbox worked fine... I tried using my router and had the same problem. The issue is I can't have both running online at the same time, there's some sort of Ip address conflict. I tried to access my router settings through my web browser because I wanted to try to port forward my xbox but i think my router is too old, I never had cause to tweak the settings till now.... If anyone has any suggestions on how to resolve this it would be greatly appreciated (I already contacted Insight Customer Service, they're only suggestion was to renew my PC's ip address, this had no effect) -Thx