polaitaly, there are programs which convert a melody line in MIDI to sheet music, simple ones being available free on the internet (ask if you want to know more).

What you're asking for is different though: it would be far more complicated for four-part harmony in MIDI to be deciphered and converted into separate melody lines on a stave, and as far as I know, no-one has written a program to do it (though I'm ready to be amazed...)

I think you will have to make do with using your ear to work out the harmonies, then writing them out if that helps. Try 3rds and 5ths then modify?