you can try using the build in vocal remover in programs like adobe audition/cool edit. it's on the channel mixer. that's the easiest way but also it worked on about five out of five hundred tupac songs. so the likelyhood it's gonna give you a cpu acapella is low. it's definately gonna alter how the song sounds. for example, on 2 live and die in l.a. it brings out the guitar and chorus vocals and almost negates everything else.
the hard way is to layer the full song with an inverted version of the instrumental, but the instrumental is normally a few % faster or slower than the retail track for whatever reason. this is a way more advanced method to go about it and it's harder to get the result you want. and unless you have an understanding of digital audio and audio waves in general, you're not gonna know what to do if you run into problems, so it's not gonna matter.