Can people stop suggesting using 6-channel sound? I think that the DVD and the driver's tests determine whether 5.1 works or not.
What you really want to check kao is if the soundcard is correctly configured to output on all jacks. I have an Realtek Avance AC'97 built-in 5.1 card myself, and the drivers came with something called AvRack, which allows you to turn the Mic and Line In into speaker outputs for 3.1/5.1. You may need to find if your drivers have a similar utility, and configure from there.
Edit: And yes I know you said Windows is set to 5.1 sound, but that doesn't necessarily mean the soundcard is too