Avid's Sibelius 7 beats all others Windows music composition products for people who can write notation.  You can mimick just about every instrument too, which is nice if you don't happen to conduct a whole symphony orchestra in your home.  Yes, there is a learning curve, but it's certainly easier than creating a work with paper, pencil, dozens of people, and expensive instruments.  If you aren't quite a Beethoven, you can import midi files or (more work) scan and import printed music.  You can then doctor the notation or instruments to complement what you want to accompany your video.  If the composer has been deceased for a while, you don't have to worry about royalties or retribution until meet the author in the Infernal Regions.  If the composer somehow avoided that fate, you won't have to contend with him anyway.

M-Audio is probably more applicable to the person who wants to create "electronica" type music in a shorter time frame, and perhaps less formal music training.

Garageband may be easier still.  There are Mac and iPad versions.  Anyone here try either?  I don't think Avid has written any Android versions for tablets.  If Avid doesn't try soon, it will be too late.