Make sure your USB device is connected to the PC. Open Virtualbox. Using the scroll bar on the right, scroll down to "USB". Click on that, and it will give you the window to create new USB device profiles. Click on the second icon from the top, "Add Filter from Device" -- the icon with the "+" sign. This will list all of the USB devices that Virtualbox detects. Click inside the checkbox of whatever USB device you want to make visible inside your Windows XP virtual machine. Click "OK" to close that window. When you click on "Start", Windows XP should start with that USB device visible to it.