Sprint Mobile Broadband + antivirus updates

A friend has Sprint Mobile Broadband service via a USB receiver. It works fine, and he needed an antivirus program - no big deal, right. Well we tried Avast, AVG Free, and the trial of Kaspersky. None of them could connect to their updates server to receive new virus definitions/program updates. Avast even kicked us offline all together.

Does anyone know what is it about the mobile broadband that is giving us trouble? I know it doesn't work like an ordinary broadband connection... but how so I don't know. He's running an HP laptop with Vista Home Premium.