下面有两种办法可以安装yum：

一、下载yum的rpm包直接安装

http://centos.ustc.edu.cn/centos/5.4/os/i386/CentOS/yum-3.2.22-20.el5.centos.noarch.rpm

http://centos.ustc.edu.cn/centos/5.4/os/i386/CentOS/yum-fastestmirror-1.1.16-13.el5.centos.noarch.rpm

下载到某个目录之后，执行下面的命令
rpm -ivh yum*.rpm
如果提示缺少什么文件的话，请看第二种方法。

二、通过rpm包来安装
依次执行下面的命令：
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/elfutils-libs-0.137-3.el5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/gmp-4.1.4-10.el5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/readline-5.1-3.el5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/python-2.4.3-27.el5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/python-iniparse-0.2.3-4.el5.noarch.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/libxml2-2.6.26-2.1.2.8.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/libxml2-python-2.6.26-2.1.2.8.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/expat-1.95.8-8.2.1.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/python-elementtree-1.2.6-5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/sqlite-3.3.6-5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/python-sqlite-1.1.7-1.2.1.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/elfutils-0.137-3.el5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/rpm-python-4.4.2.3-18.el5.i386.rpm # (1)
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/m2crypto-0.16-6.el5.6.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/python-urlgrabber-3.1.0-5.el5.noarch.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.i386.rpm
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/yum-3.2.22-20.el5.centos.noarch.rpm (2)
rpm -Uvh http://mirror.centos.org/centos-5/5/os/i386/CentOS/yum-fastestmirror-1.1.16-13.el5.centos.noarch.rpm (2)
yum -y update

有些时候单纯的某一种方法并不能安装好yum，那就两者结合起来，一切就OK了。

都是从网上找到的，谢谢高手的资源。http://hi.baidu.com/dyspangzi/blog/item/7a8154a78704c299d143588c.html
http://www.vpsad.com/html/y2010/2126_how-to-install-yum-for-centos.html