Go to the website of your computer manufacturer and download the latest version of the audio device driver and install it.
Open Device Manager (Click Start -> type Device Manager in the box, and press <enter>)
On Device Manager, go down the list to where it lists "Sound, Video, and Game controllers" and click the arrow next to that. It should list your sound card under that. If there is a red x or a yellow warning triangle, then it means that something is wrong with the device. You can try to update the driver and see if that gets it working (Windows will fiind the driver version you just installed above and use it). Also, right click on the audio card and if the list of items includes "Enable" then click that.