I've been using my Leonardo for a while now and its been working fine. I installed the drivers using these steps: http://arduino.cc/en/Guide/Windows#toc4
. Suddenly today, while I was uploading a programme the software hung up. So, after waiting for a while I disconnected the board, leaving the uploader hanging. Obviously, I tried reconnecting the board and now Windows says, "One of the USB devices attached to the computer has malfunctioned, and Windows does not recognize it".
When I go to the device manager, I see that the board is getting detected under Universal Serial Bus controllers as an unknown device. The device status reads "Windows has stopped this device because it has reported problems. (Code 43)". When I update the driver, it by selecting the appropriate arduino driver, it says "The best software for your device is already installed. Windows has determined that driver software is up to date" and it still labels it as an unknown device.
I even tried the RunOnce solution, but that didn't seem to solve the problem.