run the following command:
rpm -qa | grep el5

Thank you, Jim. That "rpm -e" seemed at first to do the trick. Now more packages are installed under that version.

I then followed the CentOS 4 instructions at http://wiki.centos.org/Repositories/RPMForge but "yum install nagios" gives me the following error.

[root@nagios-server tmp]# yum install nagios

Loading "protectbase" plugin

Setting up Install Process

Setting up repositories

Reading repository metadata in from local files

81 packages excluded due to repository protections

Parsing package install arguments

Resolving Dependencies

--> Populating transaction set with selected packages. Please wait.

---> Package nagios.i386 0:2.9-1.el5.rf set to be updated

--> Running transaction check

--> Processing Dependency: rtld(GNU_HASH) for package: nagios

--> Processing Dependency: libc.so.6(GLIBC_2.4) for package: nagios

--> Finished Dependency Resolution

Error: Missing Dependency: rtld(GNU_HASH) is needed by package nagios

Error: Missing Dependency: libc.so.6(GLIBC_2.4) is needed by package nagios

[root@nagios-server tmp]#