From the Gentoo handbook, here's an overview of Grub error codes including what error 18 means -
http://www.gentoo.org/doc/en/grub-error-guide.xml. Try following it's recommendations, or try re-installing Grub once your system is loaded after booting installed system from your CD. Simply go into YaST, Boot Loader Configuration, and write out the boot loaded again. Another alternative people use is to simply use LILO rather than Grub.