If this is something that you want to explore, and aren't sure how deep to get into, I'd highly recommend going with Garritan Personal Orchestra to get your feet wet. It comes with an intigrated sample playback engine (kontakt) and puts you an entire orchestra at your fingertips.
The question is, how do you mean to go about composing? There are some good notation programs that GPO can interface directly to, so you write the score the same way Mozart did, except with a mouse instead of a quill. GPO then can "read" your notation, and plays all the parts simultaniously according to your instrument assignments. The other method is to play the parts on a midi keyboard and have the midi stream recorded into a sequencer program. You record different parts to different tracks, with each of those tracks assigned to different instruments or sections. It's also possible to combine the two methods, with some sequencing software having the ability to enter the score by either recording a midi keyboard, by writing the notes on a staff, and a third widely available method called piano roll editing, where a simulated piano roll like on the old player pianos is used for editing, and even composing.
Your options are many, and can run from about $250 to well into a $10,000 bill. Google "notation software" , "orchestra library", and "midi sequencer".
Do check out these GPO links.......
http://www.garritan.com/audition_hr/ ......audition the instruments
http://www.garritan.com/mp3.html ......hear some user compositions
