2. THE KNOCKOUT METHOD
using 'knock0ut' you can spectrally subtract one piece of audio from another. Start by extracting the centre mono of a track (soundforge or cooledit will do it with the pan/expand feature). then try and make an instrumental out of loops from the track and 'knock them out' of the vocal parts. This can work even if the instrumental isn't excatly the same as the song.

3. OTHER METHODS
Besides these methods you can try Cool edits (now audition) noise reduction feature, which is very powerful. Analyse a bit of instrumental and get a profile (6000 or 4096FFT is a good size).. reduce than from the whole song.
You can also try the soundhack spectral plugins and Voxengo's Transmodder to futher reduce spikey nosies like drums.. (don't ask me how .. its all trial and error).


You will NEVER make a totally perfect acapella with 2 and 3. You can however make something good enough to work in your mix.. USE YOUR EARS! LISTEN and learn and you might get somewhere

SOUND FORGE

Fortunately, Sound Forge has a special Mid-Side conversion feature (normally used for another purpose) that pretty much makes this process automatic. Without it, the process would be much more involved (ie. having to remove the vocal first, converting to mono, phase inverting, and recombining with a mono version of the original file).

Here's how to do it the easy way in Sound Forge:
1 ) Open the original audio file.
2 ) Choose Edit>Select All to select all the data in the file.
3 ) Choose Process>Pan/Expand.
4 ) In the Pan/Expand dialog box, choose the Normal Mix Of MS Recording preset.
5 ) Click OK.
6 ) Choose Edit>Selection>Set.
7 ) Choose Left in the Channel parameter drop-down list.
8 ) Click OK.
9 ) Choose Edit>Copy.
10 ) Choose Edit>Paste Special>Paste To New.
11 ) Save your new audio file.

Depending on the type of material that you're processing, the results will vary, but they'll never be perfect. Anything that was in the middle of the mix will now be in your new audio file, which probably includes the vocal, bass, and some of the other background instruments. You can try to tweak the mix a bit with EQ, but other than that, what you get is what you get.


Heres A Program That Helps Create Acapellas
http://www.kvraudio.com/forum/viewtopic.php?t=79428


Another Program That Produces Various Results
http://mp3-player.audio4fun.com/mp3-music-editor.htm

And For Cool Edit Pro(I Was Told This SUmtimes Works But Havent Tried It)
effects>amplitude>channel mixer>vocal cut