Make sure on xbos you have set as static IP and als make sure you do a save and exit at bottom of screen.
Otherwise yo need to analyze your network.
1) Verify Cross cable as you are not using a hub or router.
2) verify you have correct settings on each device
3) on pc try Start-Run type 'command' and a dos window will open up, now type
"ping IP ADDRESS if xbox" and see if you get a response.
4) I think this coudl be issue as well, FIREWALL make sure turned off if you have one an dif windows XP make sure default on eis turned off for connection