pglewis is right...
Think of it like this. Before the song is mixed, say you have one track of guitar, one track of drums, one track of bass. I'll use a really simplistic analogy and say it's like having three cups, each with a bunch of rice in them.
Then you mix the three tracks into one (pour the rice cups all together into one bowl).
Once you have done this, there's no way to get the same rice grains back into the cups they used to be in. They have lost any information about where they came from and all you can do is guess.
There are some programs that try to use the fact that certain instruments are predominant in certain parts of the spectrum, and so you can do things like "remove" the vocals from a song, but really all you are doing is cutting out the frequencies where the vocal is predominant, and hoping that this removes enough so that any other effect of the vocals is so dimished that the sound like they're gone, and that you didn't also remove important content for all the other instruments in the mix to sound like they should.
Hope this makes some sense...
-AlChuck