Go to your Device Manager by right clicking on My Computer and selecting Properties from the bottom of the list, then pressing the Hardware tab.
Look down the list for any Yellow exclamation marks showing hardware that is not installed.
If there are, you need to install the drivers for your audio card by finding out first of all whether it is on-board or a PCI card, and then, what manufacturer it is.
If there are no yellow marks by your devices then I would suggest removing the drivers for your audio card through Add/Remove Programs from the Control Panel, and also before restarting, use the Right Click > Uninstall feature on your Audio Device in Device Manager.
Let the computer restart and re-detect the audio card and see whether it picks up the audio device and tries to install it. If it does let windows find the driver automatically and install itself.
Unfortunately if that doesnt work the drivers that windows has chosen arent doing their job correctly and you are going to have to start searching the net for drivers for your specific card.