I also needed to downgrade a Dell XPS 430 to XP, and found all drivers but the IDT/Sigmatel one, so I end modding a suitable one (from Gateway) to work for the 430.
It works so far and has passed my basic funtionality tests; however, no promises nor warranties with this one. If your system or computer breaks, you keep both pieces.
Please do not host this driver, link to its Download Page so that I can keep track those whose downloaded/helped 