Hello,
This is a known bug that happens when you install safari 4 beta. Are you running this?
There is no known fix at the moment, however, all you have to do is use the safari 4 beta Uninstall packet that came along with the safari 4 beta installation packet.
just follow the instructions and it will automatically install the previous version you had installed, and then restart the computer.
your MSN Messenger should return to normal.
if you trashed the application just download it again and use the uninstall packet.
Just a temporary solution until Apple fixes the safari bugs.
hopes this helps!!