look through the preferences file in your home library folder.
other than that -
you see, even with camino or whatever MSN Messenger used to always open up to IE for hotmail. It appears that Apple has made Safari appear to the system, to be IE. Therefore when MSN says it wants to open IE to use hotmail it goes to Safari.
This is taken to the n-th degree in Panther where it appears (from developer release of course) that Apple wont be shipping it with IE.