Terry, the VBoxManage command wasn't finding the machine, then after I started the Vbox manager GUI, it worked. Go figure. But, the machine booted, Windows complained about an improper shutdown, and everything is there. Beautiful, thanks!
I will cleanup my checkpoints per your advice. I was not aware that they affected performance - it seems a bit counterintuitive that they would. I had tried to delete some old ones already, but since that is disabled while the machine is running I just thought it wasn't possible at all.
Anyway, thanks a lot for the help guys. I hope in the next version, these tasks of repairing the xml, and forcing a cold boot on a saved machine using the current hard disk image, can be automated and available from the GUI. Otherwise, to the non-techie user, a VM is a very, very fragile thing compared to a physical computer, and probably not a valid substitute.