Good luck... you'll need it.

Seriously - think how difficult it is for a human to listen to a certain instrument in a piece of music - could you for example hum the 3rd Violen part of Beethoven's 5th!! I've never heard of anything that can do what you ask - and I wouldn't be too confident with the results if it did. It may be possible with a great deal of filtering to pick out individual frequencies but when so many instruments overlap the same range of notes how are you going to program a PC to distinguish between them?

Let me know if you get it to work though!

Cheers