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Try aMSN. Google it. I'm not too sure though, as I haven't tried it myself.
 
You are a little vague here. Let me try to fill in the blanks.

You want to view the webcam of someone using msn messenger on windows XP? And you want to be able to do this while on your Mac?

The only current Mac application to be able to do this is aMSN. Its interface is a little clunky, but it will do what you need.
 
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