Try this: http://arduino.cc/forum/index.php/topic,118440.0.html
I added a wrapper that will try to unlink the used comport (disabling and reenabling the arduinos via software). If problem is fixable a window will pop with a "fix" button.
Tell me if solves your problem. If you can't use the arduino in another machine, the problem is probably in the board (8u2 chip)