Following the instructions from https://help.ubuntu.com/community/Boot-Repair, I download and boot the live CD (xubuntu 12.10 64bit); install and run boot-repair:
...and start it. Get the message:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
"Please close all your package managers (Software Center, Update Manager, Synaptic, ...). Then try again."
But I have no package managers running, so I'm stuck.
I've seen that a few people have gotten this problem, but haven't seen a solution to it yet.