Alright, I assume you're running Win XP. Get to your command prompt, start, run, "cmd". "type ipconfig /all" in the prompt. Now, find where it says Local Area Connection. Beneath this will be your PC's IP, Subnet Mask, etc... Keep that window open, we're going to need to refer to this info in a sec.
1. Your Xbox needs to be on the same subnet as your PC. ie - If your PC's IP is - 192.168.0.201, your xbox should be 192.168.0.xxx You need to make sure this info matches up on your xbox.
2. Your PC and Xbox need to have matching Subnet Masks as well. If your PC's is 255.255.255.0, your Xbox must be the exact same.
3. You should set the DNS server for good measure as well. If your PC DNS server is 192.168.0.1, your Xbox will need the exact same.
Boot into X3 Config Live to double-check your network settings, make sure all the specifics mentioned above match up, and everything should work according to plan.