hhmmm. Ok you are thinking backwards now. You can connect wireless device if the computer is PLUGED into the internet.
Buying the the $100 adaptor is your best bet but you could also try
http://lifehacker.com/software/how-to/build-a-wi+fi-adapter-for-your-xbox-360-on-the-cheap-306678.php
Which is a site that lets you make your own wireless adaptor








