Oh when you tell it to reboot it will give you a message to remove the installation CD or whatever. If you used a CD then just take it out. If you used an ISO file then you have 2 options:

1. Go onto the HDD and rename the ISO file.
-or-

2. The better way is when it asks if you want to continue testing then tell it yes. The click on the shutdown X in the upper right corner. It then gives you some choices and you want to select "Power down".

Then in the VBox control panel highlight the VM, hit SETTINGS, and under the SYSTEM TAB, you'll see a box with the boot order. Move the HDD to the top of the list. Then it won't try to reload the ISO file