Xbox LIVE connection trouble... help!!!?

Im using a shared network connection between my pc(vista) and xbox 360 via the ethernet cable bundled with my xbox. the source of the broadband is a 3G modem. I've managed to establish a connection to xbox LIVE every time that ive tried to so far, but before i can move on, the xbox says it requires an update. As soon as it starts downloading it spontaneously fails and disconnects me. Can anyone help? Thanks.