audio file playback distorted

I have a Dell Dimension 4700c running windows xp sp2. Recently all media
audio files are distorted on playback. Either WMP 11 or listening to internet
radio stations, the songs play haltingly and very scratchy. I even listened
to the wav and midi files in the Windows Media folder and they are somewhat
distorted. The problem isn't with the speakers as the time line in the WMP ll
actually slows down and speeds up with the distortion.
Any clues?
Thank you