Hmmm... have you done any "sound tests" in Windows for speaker assignments? Are you certain you have your speakers correct? Maybe put the left speaker on the right, and the right speaker on the left, or if using cans, put them on facing the other way?...
I've got one computer that plays everything "backwards", when compared to all of the other computers in the house, and I've got everything hooked-up correctly, according to its speaker jacks and labels, as well as the speaker cabinet labels, so I just put the left on the right, and the right on the left. Problem solved... sort of - I think it's the sub-woofer I'm using with the set-up.
