I have the same problem in Yahoo Messenger. Yet my web cam works fine in MSN, Digsby and the viewing software that came with the cam. Please let me know if you find a solution.
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Start up the messaging program you use and see if it fires up, you may get lucky.
Fortunatly new ones are pretty cheap now.
I had an old one (chepie) in W7 beta that worked with the W7 drivers, but I had to chase drivers for Vista.