If you happen to install CentOS without X Window and then manually install X Window using yum, the newly installed X Window will not run by itself upon bootup.
So to make it run upon bootup, do the following:
1. from the bash prompt, type in (without the quote): "vi /etc/inittab"
2. look for the line: "id:3:initdefault:"
3. press i (enable insert mode in vi)
4. change the 3 to 5, so the line now become "id:5:initdefault:"
5. press escape, type in (without the quote): ":wq"
6. now you are back in bash prompt, type in (without the quote): "reboot"
