VirtualBox installation on Ubuntu

Upon installation of VirtualBox in Ubuntu, you need to compile the kernel modules needed for vbox to operate.

First, get the GNU/Linux kernel source and headers
$ sudo apt-get source linux-image-2.6.24-25-server
$ sudo apt-get install linux-headers-2.6.24-25-server

Now, compile VirtualBox modules with the vboxdrv command
$ sudo /etc/init.d/vboxdrv setup

Don't forget to add your user to the vboxusers group.
$ sudo addgroup $USER vboxusers