This tutorial is for Windows 7, for help with earlier version reply to the thread and I'll help the best I can.

This is a widely asked question amongst gamers in the Xbox world, there's serveral reason why one would want to use their laptop or computer as a wireless adapter to connect to their Xbox 360, I used to do it because I couldn't afford a proper wireless adapter and I wanted my Xbox in my room. (My Modem (router) is downstairs). So I'm going to try to go into as much detail (with pictures) as I can.

Getting Started:

What you want to do first of all is take your Ethernet cord, and plug it into both the back of your Xbox and into your PC. This is obviously needed as it will be the "bridge" of the whole connection, and your way to the internet.


Progressing:
Make sure you definitely have your Ethernet cord plugged in properly at both ends. Next, right click on your computers wireless (internet bars) icon:
(found on the bottom-right of your screen.)


Select "Open Network and Sharing Center". Once that has opened up, it should look similar to this:


Next, click on "Change Adapter Settings" on the right hand side menu. You should now see something similar to this:



Yours SHOULD NOT have the red "X" on Local Area Connection, mine shows that because I currently do not have an Ethernet cord plugged in.


Advancing:

Okay now we're in the Adapter Settings, you're going to want to highlight both your wireless adapter and Local Area Connection. Once you've done that, right click on one of them, and select "Bridge Connections"


You should see this window pop up:



Once that's done, just give it 10 seconds to sync everything up, then go to your Xbox and "Connect to Xbox LIVE". :boththumbsup: