Centos 5 is not supported running yum will produce an error like this:
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/
removing mirrorlist with no valid mirrors: /var/cache/yum/base/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: base
Solution - Update this file CentOS-Base.repo
# CentOS-Base.repo........