Hi, I haven't done this personally but found this:
Stuff you will need:
1.
Wireless router (linksys preferred)
2. Crossover cable (Looks like a Ethernet cable but it isnt you can get them at any computer store)
3. (1) Ethernet card in your computer
4. Windows Xp Pro (SP2). (Home Edition may be the same but just to be safe im using pro)
What to do:
1. Install Windows Xp pro and Service Pack 2 with all other updates.
2. Install Ethernet card (if necessary)
3. Install router (if necessary)
4. Plug in crossover cable to ethernet card and to
Xbox.
5. go to START>CONNECT TO>SHOW ALL CONNECTIONS (Can also be found in my computer>my network places> view network connections
6. right Click on
wireless network connection and click on properties
7. Click on advanced tab
8. Check the box next to " Allow other network users to connect through this computer's internet connection" then click OK.
9. Right click on Local Area Connection the click on properties
10. Click on internet protocol (Tcp/Ip) then click on properties
11. Beside IP Address type in: 192.168.0.25
12. Beside Subnet mask type in: 255.255.255.0 (Remember the this IP and Subnet mask are different depending on the brand of the router. This configuration is set for Linksys routers.)
13. Click OK
14. Turn on
xbox go into the internet settings and make sure it gets an Ip and subnet mask automatically. B/C You have already assigned it.
Hope it helps
source: anet forums