Situation: install new box (SL41) - goes v well.
LSB Version:    1.3
Distributor ID: ScientificSL
Description:    Scientific Linux SL release 4.1 (Beryllium)
Release:        4.1
Codename:       Beryllium

Post-OS load+config, yum -y update updates all - looks good.
Reboot box. All looks v well.
Then yum install few things:  mozilla lynx pine
Use box for mozilla browsing etc couple hours.
Overnight, yum crontab updates nothing (according to /var/log/yum.log)
next day: yum is broken:
********
[log in to unmask]> yum check-update
'import site' failed; use -v for traceback
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:
2.3.4 (#1, Mar 14 2006, 11:28:20)
[GCC 3.4.5 20051201 (Red Hat 3.4.5-2)]

If you cannot solve this problem yourself, please go to
the yum faq at:
   http://wiki.linux.duke.edu/YumFaq
********
from rpmpkgs:
python-2.3.4-14.2.i386.rpm
(++lots of other python stuff)
yum-2.4.2-4.SL.noarch.rpm
yum-conf-41-9.SL.noarch.rpm

via the duke wiki & google, most mentions are old (2004) & saying "python 
was not installed". But it is installd & everything worked. It ends up in 
a mire of unrelevant snippets.

using python -v for traceback:
# /usr/lib/python2.3/os.pyc matches /usr/lib/python2.3/os.py
'import site' failed; traceback:
Traceback (most recent call last):
   File "/usr/lib/python2.3/site.py", line 60, in ?
     import sys, os
ValueError: bad marshal data

Again it ends up in a mire. The only thing I can think of is that somehow
installing mozzila,lynx,pine (which have done on many other box) somehow in this 
one instance broke python/yum. Or it's a version issue?

Thanks for any fix hints.