It appears you never got a proper welcome, so - Welcome to the CentOS fora. Please see the recommended reading for new users linked in my signature.
While writing a dvd in my centos 5.5 ...
You should update to the current/supported release CentOS 5.7. See the CentOS 5.7 Release Notes
for details. By not updating you are implicitly accepting that you will live with numerous bugs and security issues (and associated known exploits) that have subsequently been fixed.