I tried to install Ubuntu 12.10 i386 with the latest VirtualBox on windows 8 x64 pro sony vaio vpcf11e1r. At the end of installation I waited for about 10 minutes observing the system is going down for reboot now! and reset vm manually. New boot resulted in the same installation procedure asking to try or install it again. Why the reboot failed?
Your VM is booting the ISO/CD Image, not the Installed system. That happens because you still have the ISO or CD in the VM drive un mount it, eject it and reboot the VM.
Source and credit to Uri Herrera from comments: