Tjclark:
When I plug in the malfunctioning arduino I get no sound at all from my computer. The LED labeled 'L' on the board blinks 3 times then stays off , and the "ON" Led turns and stays on. Other than that I get nothing
When I plug in my working arduino I get the typical windows "device connected" sound.
Oh, if there is no unknown device, it is possible that the small chip is dead (the 8u2), check in another usb port or pc to be sure, attempt to put it in dfu mode, at least to know if it answers something