it may be that grub was not installed.
I agree; also double check that the drive you installed CentOS on is the drive that you're booting from?
I've (so far) installed with the default disk layout and package
selection.
You may want to try specifying a /boot partition.
If all else fails, I've found this guide very useful http://www.gnu.org/software/grub/manual/legacy/grub.html
You'll probably want this section: http://www.gnu.org/software/grub/manual/legacy/grub.html#Installing-GRUB-nat...
Hope this helps, Rob