It depends whether this rogue engine deleted the MSN registry key. If not, then you should be able to reset it thus:
Open the search window (click search on the toolbar)
Click customize
Click Autosearch settings.
Should see a drop-down of available engines which will be set to the current one in use. MSN should be in the list too, so re-select it. If MSN isn't there, you'll have to reinstall it.