The main problem is that the recent VirtualBox 5.x series has some major compatibility problems with Windows and/or the anti-virus installed with it. Launching a VirtualBox image could result in a pretty scary error message like:
Error relaunching VirtualBox VM process: 5And it's a "hard" error, blocking the VirtualBox image from even attempting to boot the guest OS. Apparently it's something to do with "Hardened Security" . There's some proposed workarounds but none of the proposed solutions worked for me.
Command line: ......
Worse yet, after installation of VirtualBox 5.x (on a Windows 7 host) and successfully testing my Lubuntu guest image, my TrendMicro anti-virus updated overnight which created the above "Hardened Security" problem --- literally overnight!
The only reliable workaround is to downgrade VirtualBox back to the version 4.3.x series. That's a workaround for the VirtualBox "Hardened Security" problem, but it creates new version compatibility problems between the guest OS and VirtualBox... argh
But I found working workarounds.
Ubuntu* + VirtualBox Guest Additions + VirtualBox Host: Version Coordination Dance
So to avoid VirtualBox 5.x "Hardened Security" problem, which bricks the guest OS image, we have to run VirtualBox 4.3.x on the Windows host.
The new problem now is that version 4.3.x of VirtualBox's Guest Additions is incompatible with the latest Ubuntu* 16.04 as guest! You'll see an error when installing VirtualBox's Guest Additions 4.3.x on Ubuntu 16.04 if you read the compilation message in the terminal .
(* note: I actually have Lubuntu, but I presume Ubuntu is the same.)
The incompatible VirtualBox Guest Addition doesn't mean that the Ubuntu guest won't start up, but you may very well lose the ability to share folders between the guest and host OS, or the ability for the guest OS's screen resolution to be automatically set when resizing the VirtualBox's screen window.
The workaround to that is to install the latest VirtualBox Guest Additions from the version 5.x series (which is compatible with Ubuntu 16.04), and then go back to running the image on VirtualBox version 4.3.x on the host OS.
Now you might say: but VirtualBox 5.x won't run because of that Hardened Security bug, so how can its Guest Additions be installed?
Apparently you can download the Guest Additions separately somewhere to compile and install yourself. Apparently a version is also available from the Ubuntu software repositories...
For me, I just went to a Mac and installed VirtualBox 5.x, in order to install its Guest Additions on the Ubuntu 16.04 guest image --- and then I moved that image back to the original Windows 7 host machine. :)
Note that this workaround made folder sharing between host and guest work again. But the screen resolution still doesn't get auto-set when resizing the VirtualBox screen window, or when going full-screen. It's rather annoying.
The workaround for that is to manually set the monitor resolution from within the Ubuntu guest OS. But of course VirtualBox's virtual graphics device doesn't always present the proper resolutions to maximize screen real estate available to the guest OS based on the host OS's screen resolution.... argh
So the workaround for that is to use xrandr to create a new appropriate screen resolution mode line.
- Lubuntu 16.04 + VirtualBox Guest Additions (GA) 4.3.x = incompatible
Workaround: install VirtualBox GA 5.x
- Lubuntu 16.04 + VirtualBox GA 5.x + Windows host running VirtualBox 5.x = does not run if Windows + anti-virus creates the "Hardened Security" bug
Workaround: run with VirtualBox 4.3.x on the Windows host, but this has an incompatibility bug (see next point).....
- Lubuntu 16.04 + VirtualBox GA 5.x + Windows host running VirtualBox 4.3.x = okay workaround, but screen resolution will not set automatically
Workaround: use xrandr within the Lubuntu guest to create an appropriately sized screen resolution mode-line, then manually set the guest monitor resolution to it.
 See also:
- Ticket #14873 Error relaunching VirtualBox-5.0.11 VM process
- E_FAIL (0x80004005) when running Linux through Windows 8 Virtualbox
- Ticket #13187 VirtualBox 4.3.14 can't start VMs on certain Windows hosts
- Discussion of Problems due to Hardened Security
- Error relaunching VirtualBox VM Process: 5 | VirtualBox won't start
 An error like this: Ticket #13278 unknown version of the X Window System installed. Not installing X Window System drivers.