What sound card and what os?
Probably the easiest way, once you've determined the sound card, go to the manufacturer's website, and download the latest drivers for your OS. Then, open up your device manager (Win9x: Right Click My Computer > Properties > Device Manager Tab. Win2k/XP: Right Click My Computer > Properties > Hardware Tab > Device Manager Button), find your soundcard under "Audio / Video And Game Devices", and right click remove (Win9x) or uninstall (2k/XP). Then install the drivers you downloaded, follow the onscreen prompts, reboot if necissary, and that should do it.
In anycase, the manufacturer's website should have detailed instructions on how to do this.
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