EastWest Quantum Leap Symphonic Orchestra Silver Edition for Free
As murphy's law would have it I just purchased the Gold edition of this companies amazing Symphonic Orchestra software two weeks ago and now they're giving away the Silver (starter) edition free for the next month. It doesn't have all the various instruments and playing styles my version does but if you have a MIDI keyboard (a musical keyboard for the computer) you can get a free copy of their basic orchestral instrument software (apx $200 value) to use with no time limit just for answering a survey which is all about your music making/instrument buying habits. You can also use your QWERTY keyboard to play the keys onscreen but it doesn't have the same expressive qualities as a real keyboard with touch sensitivity, easily playable chords and so on.
Regardless of keyboard you will need a beefy PC to use this software, Quad Core & 4+ GB is recommended. It works on Xp-Vista-W7.