In your autounattend.xml, add the Microsoft-Windows-PnpSysprep component. Set the PersistAllDeviceInstalls value to True . Here are the details. PersistAllDeviceInstalls PersistAllDeviceInstalls indicates whether all Plug and Play devices will be uninstalled
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Read the complete post at http://blogs.technet.com/backroom/archive/2007/02/07/is-windows-vista-wiping-out-their-out-of-box-drivers-here-s-how-you-preserve-it.aspx