Now that I have my VM in a spacier partition, I have just finished installing Debian updates, which included a kernel update, so later, after rebooting, I had to install VirtualBox's guest additions for this kernel.
Before installing the guest additions you have to install the linux-headers package for the current kernel, and if you haven't done so, install the gcc compiler too.
Then click on VirtualBox's Devices menu option, and Install Additions.
Open a terminal and cd /media/cdrom
Finally, reboot the VM.