Ok, a couple questions.
1. Are you trying to "get to live" or other "real" internet sources.
2. Trying to stream media from PC to Xbox?
or both.
Assuming you want to do both, you'll need two network cards in your computer. It is theoretically possible to have your modem using usb, and you using the network card for the xbox, but I suspect this will not work well.
In order to connect the 360 to the computer they will both have to be in the same ip range. Set the ipdress on the xbox to 192.168.0.100, gateway to 192.168.0.1 If you are asked for netmask its 255.255.255.0 and the network card you're hooking it too to 192.168.0.1 netmask also 255.255.255.0
That will allow them to connect to each other.
In all honesty, go out and spend 20 bucks for a router. You'll get a far easier setup, and firewall to boot.