Go to the device manager (Start, Settings, Control Panel, System, Device Manager) and check you have no problems with your sound card. You'll see a yellow triangle next to it if you have.
If that's fine I'd try re-installing the sound drivers. If you have the sewtup discs uninstall from the add/remove and then put the setup disc in and follow through.
If you don't have the setup disc then note the sound card make/model from device manager and download the drivers from the manufacturers website.