I use a program called Noteworthy Composer that I think highly of. It won't scan the sheet music, but it's not very difficult to type in a score. You can do multiple clefs, add lyrics and notation text, and manipulate the music to your heart's content. It outputs MIDI, or you can print the score in a very readable form (it comes with fonts to show the music quite accurately). It took me a little while to get the hang of the interface, but once I did I could enter in music almost as fast as I can type words.