rhel5.3 执行 yum 提示‘No module named yum’，不熟悉 python，请指教

os:rhel5.3，执行 yum 出错（yum 是后安装的，网上找的安装包），错误信息：
$ yum
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:

No module named yum

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:
2.4.3
......

查看 yum
$vi /usr/bin/yum
$!/usr/local/bin/python2.4
import sys
try:
import yum
......

python2.4 site-packages 下是有 yum 目录的
$ls /usr/lib/python2.4/site-packages/yum
Errors.py callbacks.pyc config.pyo failover.py ......

有多个版本的 python
$whereis python
python: /usr/bin/python2.4 /usr/bin/python /usr/lib/python2.4 /usr/lib/python2.3 /usr/lib/python2.6 /usr/local/bin/python2.4 /usr/local/bin/python2.6 /usr/local/bin/python2.5-config /usr/local/bin/python /usr/local/bin/python2.6-config /usr/local/bin/python2.5 /usr/local/lib/python2.4 /usr/local/lib/python2.6 /usr/local/lib/python2.5 /usr/include/python2.4 /usr/include/python2.3

$ python2.4
Python 2.4.3 (#1, Apr 8 2015, 02:24:21)
[GCC 3.4.6 20060404 (Red Hat 3.4.6-10)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

import yum
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ImportError: No module named yum
import rpm
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ImportError: No module named rpm

问题是出在哪呢？site-packages下的 yum 不可用？