A nice little free (I think?) program for making MIDIs is Noteworthy Composer, but you must be fairly good with music theory to actually compose a piece of music.
What you're looking for is called Microsoft Music Maker. It is what they used to make those stef midis, I believe. You set a few variables in the program, and it generates a song from what you put into it. I had the program years ago, but now I don't even know if it exists. I just make MIDIs the "hard" way.
