This is annoying, here is what I found out a few years ago.
You probably have a capture card or USB camera.
Somewhere on your PC there is an application (other than
Net Meeting) that talks to your camera. Like a Haupauge
tuner card for example. You will have to look around but
find it and switch it so it can see your camera and not
something else like a Cable TV tuner. Then exit that
application and make sure it does not autoStart when you
reboot.(otherwise it will reserve the camera for itself.)
Then after a reboot got to Net Meeting, and it should be
able to find the unused capture device.
Good luck.
Kris