I don't have the PC in question with me any more (a system I was setting up for someone), but I think it was the Realtek drivers where I had to untick an option. It was one option on Vista and another on XP, but I can't remember which, so I'll give you both as roughly as I can remember them.
'mute rear audio when playing through headphones'
'force detection of front audio panel'
I really can't remember more than the basics, and it might not even be the same software that you're using, but it's worth a go.
//edit: I should make clear that this was within the Realtek control panel applet itself.