[Yum] help on running yum on redhat 8.0

[Bill Fung]

I am running yum-2.0.3-5.rh.fr on redhat 8.0. Everytime I run "yum 
check-update", I got the error message "Existing lock /var/run/yum.pid: 
another copy is running. Aborting." . Actually, I got the same error 
message on ALL yum command except the help command "yum -h".

After searching from web, I got some hints 
http://www.fedoraforum.org/forum/showthread/t-900.html but that didn't 
solve my problem. I am sure:
1) I am running as root
2) there are no other copy of yum running
3) the file /var/run/yum.pid doesn't exist

I have read thru the documentation. Anyone could give me a hand? Thanks 
in advanced.

Bill