Victor -
While Rod has a point, he maybe isn't aware that there are lots of options out there for non-musicians. There's UJam, which probably doesn't have lots of classical choices, but I haven't checked it out thoroughly:
http://www.ujam.com/
There's also Steinberg Sequel:
http://www.steinberg.net/en/products/sequel/start.html
Sequel is more of a sequencing package, which uses pre-defined loops to create custom music.
Of course there's ACID music, a looping tool, and there are many others of this type, which allow you to "compose" by using pre-defined loops, and mixing them together. Soundforge, FL Studio, and GarageBand are others.
There are also collections of music loops in styles that you can purchase and use in whatever audio software you choose:
http://macloops.com/audio/free_garageband_classical_music_loops.php
There's a whole world of music out there for the non-musician.
Joe Bourke
Owner/Creative Director
Bourke Media
http://www.bourkemedia.com