Encountering Yum error in CentOS 5.x - Traceback (most recent call last) : File "/usr/bin/yum", line 28, in ?

After I have installed CentOS 5.4, and do a yum update, I encountered an error

Traceback (most recent call last) :
     File "/usr/bin/yum", line 28, in ?
     import yummain
ImportError: No module named yummain

I tried doing yum clean all, but the error came up even for this command. To solve the issue, I've used rpm and re-install yum-3.2.22-20.el5.centos.noarch.rpm from the CentOS 5.4 DVD again and the problem ceased.

rpm -Uvh yum-3.2.22-20.el5.centos.noarch.rpm

On another note, there were some solution on Google suggesting to install yum-complete-transaction. Now that was quite painful. I did a yum install yum-complete-transaction. But it actually  meant a

yum install yum-utils