This is just a heads-up on an issue that I resolved today:
Windows Update KB3004394 (out of a couple of dozen that came down today) on a Windows 7 host stops a Windows 7 guest from starting. Uninstalling that particular update from the host allowed the guest to start again. Looking on the virtualbox.org forums (https://forums.virtualbox.org/viewtopic.php?f=6&t=65062) confirms that others have experienced the same problem, and searching more widely on KB3004394 reveals that it is causing diverse problems, reported on microsoft.com forums, and not limited to Virtualbox. Therefore Microsoft is likely to issue a fix before long. For now, I would exclude that update from being installed.