I have a Sony XA-300 too. My whole carPC is not installed yet, but I did some testing of the unit with my laptop. I found that the Sony XA-300 was recognized as a USB sound device ONLY when my head unit was using it. In other words, if I switched over to the radio or head unit's CD player the USB sound device in the XA-300 was turned off and windows would act like I had unplugged the XA-300. This sucks because winamp dies without a sound device to play to. Sometimes my laptop would switch over to it's built in sound and keep playing, but it would not switch back when I set the head unit to the XA-300's USB sound.
So, I think I will end up changing over to one of the other inputs (RCA jacks) and just use the regular audio output from my motherboard. That won't screw up Windows every time I switch between the audio sources on the head unit.
If you do figure out how to get the USB to stay "on", let me know. I am all over it. I would rather have the digital sound all the way to the XA-300.