"no boot device is available"

I've been trying to install Windows 7 and it gets stuck on booting it saying "no booting device is available" and "no operating system is installed". I tried from an iso file and from a bootable usb. I change the booting order but it doesn't resolve the problem. It sees the bootaboe usb because it disappears from the mac finder once the virtual machine starts but it doesn't boot from it. I tried reinstalling parallels as well as deleting the M and starting a new one. nothing works. Your help would be greatly appreciated.
Thanks
Annat