If I'm understanding you correctly:
1) Your DSL modem only connects via USB
2) You connected the modem to your PC via USB
3) You have your PC and XBOX hooked up to your router via RJ45 Cat5 cables.
Well, I think you have 3 options:
1) Call Bell South and ask them to replace your modem with one that uses RJ45 instead of USB
2) Get a USB to ethernet adapter from http://www.tigerdirect.com/applications/se...p?sku=TC3S-1004
and then get a USB female to female adapter from Radio Shack. Then attach your USB modem to your router.
3) Keep your current setup and try setting up your PC for internet connection sharing. Win98. 2k and XP are all capable of doing this. This way your PC will assign an IP address to your XBOX. I'm not sure if this will work but it's worth a try.
I would strongly suggest going with option 1. I'm sure Bell South won't have any problems replacing the modem. Just tell them it doesn't work and you want one with an RJ45 port. Hope this helps. Good luck.