Do you have USB 3.0 hardware?
What is the make and model of your computer?
Even if you bought an add-on controller, however, some USB 3.0 host controllers do not have XP drivers and will only function as USB 2.0 controllers. That's the case for Intel: http://www.intel.com/support/chipsets/usb3/sb/CS-033072.htm
Some add-on USB 3.0 host controllers do have drivers for XP. See, e.g., http://plugable.com/products/pcie-usb3-sp