If you are still looking..
When this happens, what is your default audio device in Control Panel >> Sounds and Audio devices?
Can't imagine this being a hardware issue but have you tried this under Ubuntu?
You could try a sfc /scannow. Just make sure that you have your Windows XP disc in case it asks for it and back-up important files just in case.
http://www.updatexp.com/scannow-sfc.html
Pauldo