[CentOS] Unable to update

Greetings!

I am running a Centos-4 box and since January 17, I have been unable to
update. 

If I run 'yum update' from the command line, I get error messages about
being unable to locate a mirror while trying to obtain primary.xml.gz.  

If I run up2date from the GUI, it times out as it is trying to access
the update file for firefox.

Other than the problem with updates, this box doesn't seem to have any
trouble accessing Web sites and there is not a proxy server in use on
our network.  Another box on my network that is running Centos-3 does
not appear to be having the same problem.

I would appreciate any hints on where to start troubleshooting this
problem.

Paul

Paul M. Wright, Jr.
Senior Systems Engineer
Northwest Fire District
520-260-7520 cellular
520-887-1010 office
520-887-1034 fax
pwright at northwestfire.org