The /boot partition isn't very large, it doesn't need to be as it only has to hold the kernel you're currently booting from (well, actually it's good to have the previous kernel too).

Over time the /boot partition will fill up with older kernels - thus reducing the available space.  It is therefore good housekeeping to have a tidy every now and then, and remove long outdated kernels.

Unlike most operations, you should not just rm them, you need to use yum uninstall as this does the job properly, and keeps everything tidy.

First you need to find out, and note which kernel you're using:

uname -a

To find out all of the others that are also currently installed:

rpm -qa|grep kernel

Depending upon how long the box has been in service, you should have quite a long list!

To remove older kernels:

yum remove [kernel-name]

<Advice from Rob Taylor>

Don't remove kernel-utils-2.4-20.el4, you need that. Would probably
recommend keeping the kernel you are using (obviously) and the one
previous to that!

</Advice from Rob Taylor>

Another way to find out which kernel you're using, and also note the previous version installed:

vi /boot/grub/grub.conf

default=1 means you're booting from the second listed kernel (and should be the same as that returned with uname -a).

You may safely use yum to remove everything other than the current and most recent prior to current kernels.  Obviously, keep kernel-utils too.