It does sound like a network problem to me. You want to make sure you have your firewalls disabled, or make sure you have ports opened to the right ip address.
First thing I would do is make sure the Windows Firewall is disabled from within the Control Panel if your running WinXP SP2. Then your gonna need to learn some numbers and make a note of them.
For example your going to need to know your routers IP Address. Which should be something like 192.168.0.1. So you need to open up Internet Explorer/Firefox and go to
http://192.168.0.1 and its going to ask you for your User Name and PW, you should find it with the documentation but its probably admin, admin as default.
Once you get into the router your going to find a Status Tab or something along those lines. What you want to be looking for are the DNS Servers. There should be 2 or 3 of those - when you find them write those down.
Now I find the best way to control a network is by Static IP Addresses. What that means to you is each computer that is within your network - or connected to that router will have its own unique IP Address. So say Computer 1 - will have an IP Address of 192.168.0.100, Computer 2 can be 192.168.0.101, Xbox can be 192.168.0.102. You just cant have the same IP Address in the same network.
You dont have to have all the computers on a static ip you can leave them as dhcp - which means it will pull an ip address from the router on its own. But for the xbox and your computer I would recommend static ip
So now that you have those DNS Servers your going to go to the control panel and double click on Network Connections, right click on your LAN connection --> go to properties --> highlight Internet Protocol --> Click on properties....Check Use the Following IP Address. Type in 192.168.0.1xx - Subnet Mask=255.255.255.0 - Gateway or Router=192.168.0.1 and the DNS Servers= whatever you found from within your router. Then just click OK and to verify you have changed your IP. You can go to Start - Run - Type in cmd hit enter - Then type in "ipconfig /all" and it will show you.
Xbox Samba Sharing Guide - This is a step by step way how to share files from your computer to your xbox, so say if you have alot of movies or music on your computer you can access them from your xbox.
I hope this helps you some. Good Luck