How can I stop the audio howling when using my webcam?

When I use my Logitech Messenger Webcam the audio howls. I can't seem to get the speaker volume and the microphone sensitivity set in a way which will make the howling go away.

I've only had the Logitech Messenger Webcam a short while, but so far I have not been able to use it for audio conversations at all. The howl is too much.

Any suggestions appreciated.

PS - I have tried moving the webcam and PC speakers around. (No joy yet.)