The other day I was just working at my computer as usual in windows xp when the ding sound that happens when you plug in a usb device, just randomly happened but I did not plug anything in. Thinking wtf is going on here I see that windows is telling me "now installing drivers for unknown device", "FOUND NEW HARDWARE - 'Unknown Device'. etc. I look again, and see that indeed, "Windows has finished installing the drivers for Unknown Device". I then clicked Finish thinking WTF??
Just happened to me exactly as you described, only its four years later and I'm running Windows Vista Ultimate x64. Not finding anything else on the web for this.