Here's one I don't understand at all! We used to be able to connect my sons Xbox to my laptop via cable, then wireless to my PC then to the ADSL and then Xbox live - absolutely no problem. While all this was going on both PC and Laptop could see each other, and exchange files. I went on a business trip, connected laptop to ISP using the modem.
Strange but true - on my return the with the network bridge enabled XBox live works fine and the laptop can still connect to the internet through the PC / ADSL ... BUT neither PC nor laptop can see each other / exchange files - to do that I have to remove the wireless from the network bridge ... but then Xbox cant connect!
Now started to lose what's left of my hair!