virtualbox.org

In the Ubuntu guest. The files you are showing are not VirtualBox.org. I would remove them from the synaptic Package Manager and then install the guest additions that came with your Host install. It is known to cause conflicts and you can not run a virtualizer inside a virtualizer anyway.

I looked a little closer using: $ aptitude search virtualbox, and got the list below:
p virtualbox-guest-additions - guest additions iso image for VirtualBox
p virtualbox-ose - x86 virtualization solution - base binaries
p virtualbox-ose-dbg - x86 virtualization solution - debugging symbols
p virtualbox-ose-dkms - x86 virtualization solution - kernel module source
p virtualbox-ose-fuse - x86 virtualization solution - virtual filesystem
p virtualbox-ose-guest-dkms - x86 virtualization solution - guest addition modul
p virtualbox-ose-guest-utils - x86 virtualization solution - non-X11 guest utilit
p virtualbox-ose-guest-x11 - x86 virtualization solution - X11 guest utilities
p virtualbox-ose-qt - x86 virtualization solution - Qt based user interf

The "p" appears to mean that there is no trace of the package on the system, so while the packages are available, I have none of them installed.

To verify, attempted to remove them, and they are indeed not installed.

Then you are good to go.

Alas, I still get the protocol error when attempting to symlink files on the shared drive.

utski wrote:user@ubuntuusd:/media/sf_test$ ln -fs item-one dir1/dir2/item-two
ln: creating symbolic link `dir1/dir2/item-two': Protocol error

After trying around a bit myself I came to the conclusion, I can't create symbolic links inside a shared folder of a Windows Host.
You can point to files in your shared folder as long as you create the link inside your linux file system.