Earlier this week my virtualbox vm's stopped working.
After failed attempts to re-install. I began searching the web but could not find much information.
I was aware a number of windows updates had run automatically just before the problems began.
Finally I found this http://superuser.com/questions/851227/virtualbox-4-3-20-stops-working-after-windows-update
so I removed Windows Update KB3004394
After a couple of restarts and some more re-installs all is back to normal.