Try uninstalling your sound card then reinstall. Right-click My Computer, choose Properties, under Hardware tab click Device Manager, under Sound, Video & Game Controllers, right-click your sound card and choose Uninstall.
Reboot.
When Windows starts, it should start looking for the driver of your sound card again (new hardware found). If Windows cannot find the driver, just insert your motherboard cd and reinstall the driver (kung built-in sya), or your sound card's cd kung add-on sya.
Kung walang "new hardware found" after reboot, baka sunog nga. But check mo muna baka for some reason na-disable sa BIOS yung sound controller mo. Refer to your motherboard manual on how to enable your sound controller sa BIOS.