You mean a notifier plugin. This doesn't notify you when a contact starts writing, but when a contact opens a conversation window with you (in other words: that's a stage earlier then starting to write)...
To see a list with current open-window notifiers see:
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CookieRevised's reply to How can you see if someone opens your conversation box"
But bare in mind that those plugins don't work 100% anymore (in fact, are very unreliable) because of the new ways MSN Messenger 7 uses the MSN Messenger protocol...
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