Im trying to install and run ubuntu server on oracle virtual machine?

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http://askubuntu.com – I have vm installed on I have downloaded the iso. When I run the server and select the iso file I get the error message; Failed to open the optical disk file C:\Users\Owner\VirtualBox VMs\Server4\ubuntu-12.04.3-server-i386.iso. Could not get the storage format of the medium 'C:\Users\Owner\VirtualBox VMs\Server4\ubuntu-12.04.3-server-i386.iso' (VERR_NOT_SUPPORTED). Result Code: VBOX_E_IPRT_ERROR (0x80BB0005) Component: Medium Interface: IMedium {05f2bbb6-a3a6-4fb9-9b49-6d0dda7142ac} Callee: IVirtualBox {fafa4e17-1ee2-4905-a10e-fe7c18bf5554} Callee RC: VBOX_E_OBJECT_NOT_FOUND (0x80BB0001) (HowTos)