What happened with older kernels? Or/and backup of the grub.conf file that you had before the update?
Yuo will need to boot from Installation CD using "linux rescue" and when booting is finished and filesystems are recognized you will have to copy older kernel to "/mnt/sysimage/" folder, run
and then install older kernel you from that rpm with
rpm -ivh kernel-2.6.9-xxxxxx.rpm
rpm -Uvh kernel-2.6.9-xxxxxx.rpm
if install does not complete.
Then optionally backup and then edit (still from chroot) /boot/grub/grub.conf to boot older kernel by default.