You'll find some sound card programs come with this feature. I've seen mostly onboard sound chips though to contain them mostly, but there are a few cheaply made sound programs that could do what you want.
Though, I think these programs are built for what the sound card can do, and so it might be hardware built, though software should be available that are free.
By any chance do you have Winamp?
If you don't, Winamp is a great music program, rather like Windows Media Player, but is better than that in some ways.
Now, you can download plugins for this music program to add alot of extra functions and abilities. One of them, being a vocal remover which I've just found
All these are free by the way.
I'll give you the link to winamp (It may take over the sound program you use already to listen to music) :
I'll also give you the link to the plugin:
The plugin after installing Winamp, you save in the "Winamp/Plugins" directory. Wheny you then open Winamp, you go to Options, Prefrences, and then scroll down to Plugins, and you'l find the vocal remover under "DSP/Effect".
it also comes with a slider which chooses the mix value, so you could have the vocals half off or fully off or whatever you choose to do
I hope this helps
I've just used it, and the quality of the song drops a bit but its still good if theres alot of vocals to remove which is expected, and it does take out the voice quite alot, so i'd try it.
I guess it'll also depend on the track used. I've tried it with Avril lavigne, and I think it is removing it, but its keeping the back vocal track since its lower in volume for harmonizing, which I guess is okay.
You could try this Direct X plugin of the same plugin as well which will work in programs like Calkwalk and Cool Edit which may let you remove the vocal with more precision if you have any of the programs listed: