virtualbox.org

Sometime last year, I made several Windows 2000 and Windows XP VMs using VirtualBox and backed up the entire VirtualBox directory to an external HDD. My computer was killed around last August so I had to reformat everything, and when I recovered my external HDD, I reinstalled VirtualBox, then overwrote the directory with my old one from 2011 containing the VMs.

None of the VMs were able to function. Why is this, and how do I regain access? If I can't, how do I safely back up information on a VM?

If these are v4 VMs (VM folder contains a .vbox file), then simply copy the VM folders back to wherever you want to run them from, and add them to the UI one at a time using Machine|Add... or simply double-click the .vbox file.

As to why your earlier attempt failed, that is impossible to answer with the information given. No doubt you got the folder structure wrong. I assume you realize that in VirtualBox v4 and later, Windows hosts, the default home for VM folders is <userdoc>\"VirtualBox VMs", while the default location for the VM register is VirtualBox.xml, stored in <userdoc>\.VirtualBox.