Although Windows XP is officially not supported for the Skylake platform, enthusiasts working in the mainboard industry found a way around the compatibility problems anyway. A last hurdle some people struggle with is how to enable the ASMedia USB3 drivers.
In a forum thread posted by Barbonenet, you can find an easy picture tutorial on how to install the USB 3.0/3.1 driver on an ASUS or ASRock motherboard. The guide includes two downloads. First of all of course the USB drivers and secondly the USB.SYS file which would normally be installed during the OS installation when USB devices are detected, but which fails on Skylake. After completing the full installation of the drivers, it is advised to use a USB HUB to use multiple USB devices.
You can find all relevant information in the forum here. Enjoy!