Anyways, my suggestion is to check all the drives in your system for said files as mentioned. If they are there, but not where grub is looking for you may have to make changes.
My system started its life with one drive and CentOS 4.3, and has been "yum updated" and is now at CentOS 4.5. I also added one additional drive to it over the last 18 months. The only time it has failed to boot was when I accidentally initialized the /boot partition. I believe this was an anomaly (something corrupted the MBR). I will soon find out when I do the "yum update" to the other 27 CentOS systems I manage if the update is indeed the culprit (I doubt it).
Alfred