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After having a quick look, it's possible that HP don't support Windows XP on the G61 series. They don;t seem to have any Windows XP drivers for it, in which case you are stuck. HP drivers are custom made for the custom-made hardware, which means you can only get them from HP.
You should have checked their website before downgrading the OS. Or if you did check and chose to downgrade anyway, that wasn't a wise decision.