After installing "CentOS 5" for the first time you should always update the program packages for safety reasons.

Before updating you MUST check the Kernel since the server might not be accessible after rebooting. 

The command:

uname -a

should display the following Kernel version (or above):

2.6.24.2

for the server to boot successfully after a reboot.

To conduct an update of the program packages without downgrading the Kernel to a prior version enter the following command:

yum -x kernel update

Please reboot the server after the update to check if he is starting again. Afterwards use

uname -a

to check the Kernel version again. It should be the same version.

If you know exactly that the Kernel you want to update is newer than the current one you can conduct the yum-update without the -x parameter.