I had downloaded a bad windows ISO container - it would not boot under VirtualBox.
But with some Ubuntu tricks - I recreated a new ISO and got Windows 7 up and running.
Ubuntu Mount the drive
I booted into Ubuntu (It as a VM running on the mac) and with the Mac drive mounted as a shared_folder did
sudo mount -o loop /dir/bad.iso /cdrom
1 ISO all mounted...
This is a command with more options than I think I have ever seen before.
but with some Googleness .... I found
mkisofs -udf -v -b BOOT/ETFSBOOT.COM -no-emul-boot -hide boot.bin -hide boot.catalog -o /media/sf_Mac/new.iso .
Back to VirtualBox
Create machine -> Select new.iso -> boot