I'm looking for a low cost, or open source, method to perform a video
conference.  The format would be one high end web-cam, which I am
mounting to a tripod, with video and audio streaming of the
speaker(s).  In addition I would like the ability to display a
PowerPoint presentation as part of the feed, in other words, the
speaker and the presentation need to be displayed on the screen at the
same time, as the speaker advances through the slides, the users
should see this transition.

I have considered using NetMeeting, but I do not think the solution
will scale very well.  There could be as many as 50 users connecting
to the "e-meeting".  All remote participants would most likely be on a
high speed connection, within my companies local network.