Somehow I broke yum on CentOS 5.5. I was getting this:

There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:

No module named cElementTree

Please install a package which provides this module, or
verify that the module is installed correctly.

It’s possible that the above module doesn’t match the
current version of Python, which is:
~~~~~~

If you cannot solve this problem yourself, please go to
the yum faq at:

http://wiki.linux.duke.edu/YumFaq

I was able to fix it following this:

http://stuff.nekhbet.ro/2010/05/27/yum-error-on-centos-5.html