If you are not an official MSN betatester i suggest you uninstall the one you are using now and download the public beta again. If that won't install after you done a reboot after uninstalling new beta, remove all folders pointing towards MSN Messenger, reboot...then try again.
If you are an official MSN betatester then you'll have to live with it i suppose, there are some compatability issues with the current Plus Beta (see
Patchou's reply to is there gonna be a full compability fix for the new msn beta 7.0.604?).
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