Ok all you do is get your laptop (which should have wireless internet) and a ethernet cable and plug one end into your xbox 360 and the other end into the ethernet port on your laptop.

First you go to your Control Panel>Network and Internet Connections>Network Connections

Ok you'll see two connections it should be 1.) your Wireless Network Connection 2.) your Local Area Connection

Now Highlight both of them with your mouse and right click and you should see >Add to Bridge or something with bridge in it.

Click it and it should be setting it up for you (it'll take hardly a minute)

Then you should be good (Your internet for your computer might stop working though all you have to do is just highlight them again and remove the bridge and just add it when you play xbox live)

Its better than buying that $100 wireless adapter ;)

Good Luck Hope it works