Setup Stereo Output from XP to 5.1 Speakers

OS: Windows XP
MB: Gigabyte GA-P35-DS3L (with Realtek HD Audio)
Audio Connections: 3x 3.5mm stereo plug into 6 RCA inputs on receiver

I've been trying to setup 5.1 audio from my PC to my receiver. I have all the latest drivers.

My problem is that When I try to play a (stereo) MP3 file, I get sound from all 5 speakers. I know it is not because my receiver is doing any sort of post-processing. I read somewhere that XP automatically does this "speaker fill" feature. Is there anyway to turn this feature off? When I play a stereo file, I just want to hear audio from my left front and right front speakers.