This is going to sound wierd, but can anyone direct me to a program (with instructions) on how to remove the instrumental part of a song, leaving the vocals? And the other way around?

If I am not mistaken, that is not possible unless you are in a studio with all the right equipment. I am almost certain that it cannot be done in any other way.

There are programs such as Adobe Edition (Cool Edit Pro) that have an effect to cut out the vocals of a song. The effect is called "Vocal Cut." To be honest, it isn't that great. If you want an acoustic guitar instrumental from a song that you like, it's not half bad..but for mostly every other type of instrumental it will take out an instrument or two along with the vocals.

If you are still interested, find the program, open your desired song you want to edit, highlight the entire section of the song, go up to "Favorites," and finally select "Vocal Cut."