Exactly what soundcard and what set of speakers do you use?
I had a problem awhile back that I couldnt use 5.1 from an older creative live soundblaster. It sucked bad! I upgraded to a used soundblaster audigy 4 and with the latest driver it works fine. I obviously have to disable all onboard sound options and remove all those drivers so there is no conflict. Also the speaker system I use has a blend button that has to be disabled.