It's possible that either another program is already using your webcam. Make sure other applications that use the webcam are shut down before running NetMeeting.
It's also possible (but less likely) that your webcam is not supported by NetMeeting. I say less likely because if the operating system recognizes the webcam and it works in other apps, it should also work in NetMeeting. What kind of cam do you have (brand/model/connection type)?
There are also alternatives. You can use Windows Messenger for text, voice, etc., and use a companion application along with it for video. Logitech has a really nice, FREE, application that works with any webcams.
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Jeff Hester aka "Mister BigBlueBall.com"