Have you tried 'customize' from the Search pane? that should allow you to change or reset MSN.
Otherwise I think its a registry edit. There's some info here (and probably elsewhere) for malicious resetters of your search prefs:
http://www.guardian.co.uk/online/sto...634530,00.html
You can set your default search to be google from here
http://www.google.com/options/defaults.html (downloads a registry entry) which may override your current unwanted settings.