This weekend, I tried to install CentOs 6.3 in VirtualBox VM 4.1.20. I didn’t find any problem with installation until the first reboot on which the system told me:
FATAL: INT18: BOOT FAILURE
I tried many times to re-install this Centos. Still has same problem.
I googled it and found the answer here.
As Aymen Medimagh mentioned, make sure to remove the installation media from CD-Rom or DVD. For me, I remove the ISO file. Then it works without INT18 error.