I have essentially the same onboard soundcard as you and I have a 5.1 setup, though it is a bit wacky.
My VIA Enhanced AC'97 card has an option called "smart 5.1". you can plug in four speakers into the audio in and out slots, and the center channel goes into the headphone jack. Usually it works.

You should try messing with the control panel settings. Go into the control panel and select sounds and audio devices, click on the volume tab and click advanced in the speaker settings area. select 5.1 surround sound in the drop down menu and hit ok.
if you're on windows 98 you would select the game controller/sound/multimedia control panel instead.

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