Does your motherboard come with a built-in soundcard? If you did not purchase one, but still have the three ports (usually one green, one pink and one black) on the back of the CPU, that would be the case. Take out the driver CD for your motherboard. The drivers for the soundcard are contained in it...
If you dont have a CD, you can download it from sites such as
www.driverguide.com etc. If you dont know the motherboard model, you will need a util such as Everest or Sisoft Sandra to find it out.
Arun