Even though CentOS is based off Red Hat Enterprise Linux, you cannot simply install CentOS on RHEL 5 as an add-on or surface layer. CentOS will overwrite your RHEL installation completely. However, you can add CentOS to RHEL 5 as a virtual machine. This will enable you to use CentOS and retain your Red Hat Enterprise Linux desktop.
Go to Redhat.com and log in to your account. Enable your computer for virtualization in the System Properties settings on your account. Without doing so, you will not be able to install the virtualization packages.
Open your RHEL 5 desktop command line, type "su -" and press "Enter."
Type "yum install kvm" and press "Enter."
Type "yum install virt-manager libvirt libvirt-python python-virtinst" and press "Enter."
Download a CentOS ISO and place it in the "/tmp" directory on your RHEL 5 desktop computer.
Type "virt-manager" and press "Enter" to open the virtual manager. Click "New" to start the setup wizard.
Type a name for your virtual machine and click "Forward."
Click "Fully Virtualized" and click "Forward."
Click "Local Install." Select "Linux" from the OS type drop-down menu and "Red Hat Enterprise Linux 5.4 or Later" from the OS variant menu. Click "Forward."
Click "Forward" at the next few screens. You can change the options on the screen, but the default selections are generally the best. Click "Finish" at the last screen. CentOS will start to install on your RHEL 5 desktop.