I recently upgraded my kernel which broke VirtualBox, I got the following error:

Kernel driver not installed (rc=-1908)

The VirtualBox Linux kernel driver (vboxdrv) is either not loaded or there is a permission problem with /dev/vboxdrv. Re-setup the kernel module by executing

‘/etc/init.d/vboxdrv setup’

as root. Users of Ubuntu, Fedora or Mandriva should install the DKMS package first. This package keeps track of Linux kernel changes and recompiles the vboxdrv kernel module if necessary

Usually this is fixed by running:

sudo /etc/init.d/vboxdrv setup

I then found this post which applied to the generic kernel and indicated that I may be missing the kernel headers.  Since I am running VirtualBox in Ubuntu Server 9.04 I ran:

sudo apt-get install linux-headers-2.6.28-14-server

and then

sudo /etc/init.d/vboxdrv setup

Working again.