There's no perfect way to remove vocals from a track once it's been mixed down (i.e. from anything you'd get off a CD, mp3, cassette tape, etc).
The most commonly used method of remove vocals makes use of a "standard" mixing style, which leaves the vocals centered between the left and right channels, while panning the instruments. The trick is to (warning, some physics involved) invert one of the channels, then sum the channels together, letting the destructive interference get rid of the vocals. [/technical description]
The problem with this is it can also get rid of some stuff you do want, so the track may sound a little distorted. It can also not work at all, depending on how the song was originally mixed, or if the track is mono.
If you want to try this out, you can do it manually using the freely available
Audacity* audio editor, using the available
video guide. Alternately, there is a freely available
plugin for
WinAmp that will do it for you.
Good luck
--Ryan
*The Audacity site seems to be down right now, but you can still get the software
here