Vista - Problem transmitting audio and video in WLM video calls

I have a PC running Windows Vista with the latest WLM. I would like to make a
video call with my father-in-law whose PC runs Window XP and also has the
latest WLM.

When I initiate a video call, I can see and hear my father-in-law, but he
can't hear or see me. I have a TV tuner that I disable before running WLM. If
the TV tuner is disabled, on my PC I see a window with my image and a window
with my father-in-law in it (otherwise, if the TV tuner is enabled, the
window that should show me is blank on my PC). On his PC he sees white noise
in the window that should show me.

I know for a fact that WLM video calling is basically working on my
father-in-laws PC, as he can make video calls to other people, and also I
could make calls to him with my old, now defunct PC.

I think this is probably a permission thing on Windows Vista, rather then my
Norton firewall, but I don't know how to fix it. Does anyone have any
suggestions?