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I had to reinstall my Win Vista Home Premium x 64 in which I had installed virtual box using xp as the guest OS. I had the pc backed up and can see the vdi files in the back up. Now I have installed virtual box 3.0.8 and would like to move those files into virtual box 3.0.8. Please advise how I can succeed in doing this.

Open the Media Manager and add the VDI files. Then you create new VMs and attach the existing VDIs to the VM. In case of Windows VMs, you have to match the settings it was installed with, or it will crash with a BSOD or trigger an activation. Linux Guests will get a new network interface because the MAC address of the NIC changed. You have to remove /etc/udev/rules.d/70-persistent-net.rules and reboot the Guest. The actual file and location may differ per distro.

How do I attach the vdi files after I have used the media manager to add the hdd's?

Did you even bother to look for it yourself? You get the question automatically when you create a VM

what I need to do is restore my virtual box after I had to completely reinstall Win Vista x 64. I have files that were on the pc backed up in a couple of places but I do not know what to do to restore them. I thought that I could move vdi files and that would work but obviously it does not!

If you got a backup of your entire .VirtualBox folder from your profile, then you simply restore that folder to it's original location and all your VMs should be there, with the same settings and all.