I got a strange error message when I tried starting Ubuntu after upgrading to latest virtual box 4.0.6..
At last I was able to fix it by editing the settings XML file manually
Go to c:/users/
change the line having enabledEhci=true to enabledEhci=false
did the trick !
Ubuntu – Implementation of the USB 2.0 controller not found! after upgrading to virtual box 4.0.6