I'm creating a virtual machine to use for my upcoming training courses to get around issues trying to get machines set up in a hurry on the day. I haven't used VirtualBox before but since it has free clients for Windows/Mac/Linux I thought I'd give it a go.

I found that when I tried to install the virtualbox package through aptitude, I got this error:

First Installation: checking all kernels...
Building only for 3.0.0-12-generic-pae
Module build for the currently running kernel was skipped since the
kernel source for this kernel does not seem to be installed.

Closer inspection showed that although I had the generic headers for this kernel, I didn't have the pae version (I only upgraded this machine a couple of days ago which doesn't help). To add the headers I needed, I simply did:

sudo aptitude install linux-headers-3.0.0-12-generic-pae

I was then able to install virtualbox successfully, and create a shiny new Xubuntu VM for my course attendees to use later in the month.