virtualbox.org

I had a virtual machine on a cloud server that I would like to boot locally. I have the last backup of this machine, which is simply a .tgz of the entire HD. Is it possible to restore this as/into a virtual machine in VirtualBox? I thought of simply creating a new similar machine (Ubuntu server x64) and then writing over the filesystem completely, but am not sure if there is any VirtualBox data that I need to be aware of that I would be overwriting. Can anyone point me in the right direction?

If it's just a disk image instead of a proper VM backup, then no, you can't simply "restore" it. Instead you'll have to create a new VM and mount this disk in it. It will be up to you to get the new VM recipe approximately right. You should expect networking problems since the details of the old NIC (including e.g. MAC address) have been lost.