This is even theoretically impossible unless you have access to the original recording with one track per instrument (or at least one vocal track and one instrumental) and I think digital sound does not change this
That said there are examples where this has been done with varying success. If you want to have anything near hifi I don't think it can be done - there will always be residues from the instruments...
This is way more complex than noice reduction and removal of clicks and other unwanted sounds
Audacity is actually one of the more competent programmes
I've discussed this (long ago) with a highly qualified sound technician but I'm beginning to forget most of it
This win programme claims it has vocal removal - the opposite of what you wanthttp://www.wavosaur.com/features.php