Just out of interest as I was looking at getting one of these displays:
I though these came with the resistor enabled for external power. If so, plugging in USB should do nothing. It's only a problem when you LEAVE the external supply resistor in line AND you install the USB supply resistor - IE both resistors installed is a problem, but that normally MAY damages the USB port on the PC, not the LCD?
Did they ever work? Have you contacted the supplier and priced a replacement board?