Sound problem when using 'Video' on Messenger

I bought a Logitech QuickCam camera and installed it on Windows XP system. When I tested it offline it works fine. When connected via MSN Messenger to another user on broadband, the picture is good. The sound often works OK for a few seconds, then there is strong sound interference which means we cannot speak to each other. Perhaps it is negative sound feedback but we have tried using headphones and changing volume levels without success. I had a go at searching out possible errors and the only thing I did was to follow instructions by enabling UPnP. However, this did not help.
Any help would be appreciated.
Jerry Sherriff