I looked into this a few weeks ago, I have a feeling it depends on what version of the Xbox 360 dashboard you are running. If you are running the pre-NXE (New Xbox Experience) dashboard, you should be fine with the solution Aus_Karlos posted. But if your 360 is running the NXE (with those little annoying Avatars), then you would have to upgrade your OS to Windows Vista Home Premium or Ultimate. This is what I suspect, read the links in the thread below:
Conecting XBOX to PC via Wireless router