Not counting regional ones, currently there are only three search engines:
Google (powers AOL, Netscape, lots of other stuff)
Yahoo (powers MSN, Alta Vista, CNN, some others)
Teoma (powers Ask Jeeves)
Nobody knows exact numbers but very roughly the Google group is about two thirds, the Yahoo group about on third, with Ask Jeeves about one or two percent.
So in answer to your MSN question, right now they use yahoo data… although they use only some but not all of Yahoo data. MSN’s new search engine will come out next year sometime (probably).