Cannot register the DVD image '/usr/share/virtualbox/VBoxGuestAdditions.iso'

Came across this lovely error today, when updating VirtualBox on an old laptop from 4.1 to 4.2:

Cannot register the DVD image '/usr/share/virtualbox/VBoxGuestAdditions.iso' {20f4b244-eb29-4082-ac34-4acf6ccb4904} because a CD/DVD image '/usr/share/virtualbox/VBoxGuestAdditions.iso' with UUID {62ebaac7-ba6d-4d7d-876b-362642de4c1b} already exists

Was like, "sh*t, my VM's are dead"!

Edit the '.vbox' file for the VM in question:

$ sudo nano -c /path/to/virtualbox/vm.vbox

Delete any lines that are located between the tags:

'<'DVDImages'>' and '<'/DVDImages'>'

Save the file and start your VM.

HTH