You may be stuck with buying a separate sound card, like a Creative X-Fi. Don't get one of the Audigy series, as Creative only supports Vista with the X-Fi series. Hate to say it, but the Realtek on board audio doesn't handle Vista well at all.
I also have a A8N32-SLI Deluxe and the on-board sound was a drag on the system. When using it, it runs off the cpu, so that if the cpu starts to get loaded up, everything slows down. Add that in to the fact that you've got a 3500+ cpu and things slow down in a hurry. If you haven't overclocked the cpu, you might try that and see if it helps.
Odd, the windows updates appeared to update the windows driver system.
Seeing how all stuttering just vanishes with one of the updates.
Perhaps there are better drivers for your sound card from Nvidia.
www.nvidia.com always has the freshest and certified drivers. Motherboard makers are usually relatively slow to get these drivers tweaked.
And usually the mobo tweaks involve renaming stuff, and no real tweaks.