Alright...Heres the lay down, I just faced this same problem like two weeks ago:
If you don't want to buy the Microsoft network adapter for a whopping $100, then unfortunately there is only one other work-around that I've found so far. You need a wireless access point. A USB adapter won't work for some reason, I tried it out with one I had and could not get it to work at all. So I opted for the access point. I picked up a Linksys Wireless-G Game adapter that said it would work for Xbox, and ps2 for about $20 on craigslist. The nice thing about this device is that it immediately began to get signal, and show that it got signal in Xbox 360 panel. Unfortunately it wasn't able to pick up an IP address. To fix that, I simply entered in a static IP, a static DNS, and Gateway. Then I used the supplied CD to access the adapter's firmware, and set the default wireless connection to the one that's in my house. Everything works great! Better yet, I saved $80 that I used towards some new games!