sony VGN-FZ490 audio -headphones problem under windows XP

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I installed windows XP on my sony VGN-FZ490 laptop which comes with
vista pre-installed and is not compatible with XP. All seemed to work
well except for some minor issues like the FN keys but today i
discovered that if i plugin my headphones, the sound keeps coming out
of the laptop speakers (as well as the headphones).
I don't know if this is a driver issue or a hardware issue, as i never
tested audio under vista i installed XP all through before using the
laptop with vista. I think the laptop comes with sigmatel audio
chipset and sound reality sony drivers.
if this is a driver/software thing, would installing other drivers
such as realtek work or would it screw things up even more?

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