I have an original Arduino Uno (made in Italy... supposedly) and I had the same problem.
It seems to me that something on the board was corrupted after I did some stuff to it. I fixed it by using zadig software.
http://zadig.akeo.ie/
selected the options> list all devices > select Arduino Uno from list> select WinUSB(v6.1.7600.16385) from the list and "replace driver"
I then went to device manager and installed the driver from the list of available drivers on my computer as previously suggested on the thread.
The board works fine now. I hope this helps if all else failed.