Go to the manufacturer's web page, download the proper driver - install it.
You must know what sound card you have. Look on your receipt/invoice for
the computer - ask the people you bought the system from, open it up and
look, etc.
There are applications out there that can identify what hardware you have,
but I think they all depend on a properly installed system - as I think they
pull the information from the registry. Perhaps they do not - so I will
recommend you try one..
Once you know what you have, what next? Go get the latest driver for your
hardware/OS from the manufacturer's web page. For example, let's say you
have an NVidia chipset video card or ATI video card, perhaps a Creative
Labs sound card or C-Media chipset sound card...