At work we have Windows workstations, but we develop for Linux (don't
ask; in my previous mission in another MegaCorp we had a similar setup
for admining Linux servers...). We have access to devel machines via
`ssh` and Samba, but the setup is laughable. I won't go too much into
details because it's embarrassing and because I signed some kind of NDA
somewhere.

Thing is, I setup a VM with VirtualBox in my workstation, installing a
barebones Debian Sid. To have better integration with the Windows host I
decided to install the VBox Linux Additions, but for some reason it was
not setting up the video side of it. The error message is the one from
the title:

    Could not find X.org or XFree86 on the guest system. The X Window drivers will not be installed.

Thanks to 
[this post](http://paulsiu.wordpress.com/2007/11/05/running-zenwalk-as-a-linux-guest-on-virtualbox/)
I managed to quickly find out the reason. The step that actually tests 
and installs the x.org drivers is called like this:

    /etc/init.d/vboxadd-x11 setup

If you run it with `sh -x` you will find out that it actually tests two
things: the existence of `/usr/lib/xorg/modules`, which you can either
create or just install the `xserver-xorg-video-vesa` package, and it
tries to run `X`, which you will find in the `xserver-xorg` package.

So, TL;DR version: Just install these two packages:

    sudo apt-get install xserver-xorg-video-vesa xserver-xorg

Now all works.