Problem with CentOS ~~~
Default is [zayyaroo@localhost ~]$
When we install package , we must use "yum" command ... In there you need to be root to perform this "yum" command ..
It means [root@localhost ~]#

[zayyaroo@localhost ~]$ yum -y install xinetd
Loaded plugins: fastestmirror
You need to be root to perform this command.

[zayyaroo@localhost ~]$ su -
Password: .......TYPE PASSWORD HERE.......
[root@localhost ~]#

That's all!

More info => http://www.basiclinuxcommands.com/2010/09/be-root-to-perform-this-command.html