You can only install via download link instead

rpm -Uvh http://mirror.centos.org/centos-5/5....3.el5.i386.rpm
Centos + yum on 64bits

Code: 
rpm ?import http://mirror.centos.org/centos-5/5....G-KEY-CentOS-5
rpm -ihv http://mirror.centos.org/centos/5.3/...1.2.7.i386.rpm
rpm -ihv http://mirror.centos.org/centos/5.3/...2.7.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....6-5.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....el5.noarch.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....2.1.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....5.2.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....0-2.noarch.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....6-2.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....el5.x86_64.rpm
rpm -ihv http://mirror.centos.org/centos-5/5....s.1.noarch.rpm
Then finally you can execute yum update command to update the rpms.