well if you want to remove a lead vocal from a stereo mix, it may be possible
if its a vocal that is panned dead center. but the main conditions are if:
1. lead vocal only on a stereo mix
2. not too much reverb/digital effects (1960 to early 1980's stuff)
results will vary, but sometimes you get to eliminate the vocal but still
retain a sizable portion of the instruments.
sound forge has its channel converter (remove center material from stereo mix) and most audio editors will have it as a dx/vst plug in
here are some links:
http://www.sweetwater.com/forum/showthread.php?t=4102now if you want to isolate guitar solos or bass runs, thats another matter,
however there's no harm in trying...