I have been running VirtualBox for some weeks with no problems. Now when I start it up I get this
Kernel panic - not syncing: Attempted to kill the idle task! atkbd serio0: Spurious ACK on isa0060/serio0. Some program might be trying to access hardware directly.
For reference the ISO is Slacko Puppy Linux.
I tried removing and re-adding the hard disk as has been suggested.