Installing VirtualBox guest additions in a Debian guest

Heres' how I got VirtualBox guest additions installed in a Debian guest.

First you have to install the Linux kernel headers, gcc and make, which can be done with this one-liner (run as root):
apt-get install linux-headers-`uname -r` gcc make

Then mount the guest-additions by clicking "Devices" + "Install guest additions" on the VirtualBox menu:

Shortly after, a drive will mount on the desktop containing the guest additions:









open a terminal and, as root, run the appropriate installer for your system, like f.x:
sh /media/cdrom/VBoxLinuxAdditions-x86.run

Now the guest additions are built and installed. When it's done, just restart the virtual machine.