Have you tried reading the Xbox networking tutorials?
I posted this earlier:
This will work for you REAL quickly.
(assuming you have Windows XP):
Connect a crossover cable from the PC to the Xbox.
on the PC, right click My Network Places - choose properties.
Find the ethernet adapter for your broadband connection - you should have two in total. 1 will be your broadband so get the right one. Right-click the correct one and choose properties.
click on the Advanced tab and under Internet Connection Sharing, put a tick in 'Allow other network users to connect through this computers internet connection'.
Click ok - it should set your PC to Xbox Ethernet connection IP address correctly for FlashBIOS (and future network usage).
Boot flashBIOS and select Network Flashing. You should now be able to access the IP address given from your PC (using
http://[ip address]).
This should be enough information to get you up and running.
Martin