This happened to me when I downloaded a CMedia sound update from Windows Update software update section. In the end I reinstalled the driver from soundcard manufacturer website. You probably could just delete the particular update as an alternative (in Settings then Add/remove programs). You can find out manufacturer from motherboard website or from card if it is separate. This was at least a year ago though so I am surprised if you are getting problem now, but if you never requested the download till now maybe its it.
