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

Thanks to this post I managed to quickly find out the reason. The step that actually tests and installs the 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.