Can't start guest in Virtualbox


I hope someone here knows something about Virtualbox. I am running W7 Pro x64 as the host. I am trying to install a guest on the Virtual box. I have set the guest up with installing extensions. In Storage the IDE Master shows the OS vdi and I set the host drive to my optical drive. I created the guest, but when I try to start the guest to install the OS on it I get the following message: FATAL: Could not read from the boot medium System halted.

Twice before I have removed the VM and created a new one and every time I have this same problem. Does any know anything about Virtualbox and how to solve this problem. Thanks.