You can't accurately remove lyrics from something that has the tracks mixed together, like your standard MP3/MP4/AAC/FLAC/OGG/WTF sound file. It's like you have a bucket of green paint and you're wanting to find out how to pick out all the yellow bits and separate them from the blue bits. Any "instrumental" versions of a song
are versions that never had the lyrics added in the first place.
You're free to try it, if you want.
Get winamp if you don't have it already. It's free.
Install this plugin into it.
http://www.analogx.com/files/vremamp.exe
Then, before you go to play the song, open that plugin from winamp (press control+P to open the preferences and go to the effect plugins section and start it), and then, in the preferences of winamp, set the output plugin as "discwriter".
That'll save the edited version on your harddrive as a .wav file when you play the song.
Be sure to set the output plugin back to whatever it was before when you're done.
Depending on the song, this may range from "good enough" to "didn't do jack shit". It's just how things work.