Boot the disk from under VirtualBox. You'll need to do this from another installation (or perhaps Live -- otherwise the disk won't be available for VirtualBox to control) and you just add the disk (you can add real disks) as the primary drive.
Anyway, once you're in, VirtualBox will let you take a screenshot.
Otherwise you could use a video-out (s-video or composite, whatever, if anything, your computer supports) and another computer with a video capture card. You can use VGA if you have a VGA capture card but most people don't. Then just capture all the video output from the first computer on the second. Bit of a task if you ask me but it's worth it for serious video recording.