If CentOS has already been installed without a graphical interface, you can install this with the following commands:

yum -y groupinstall "Desktop""Desktop Platform""X Window System""Fonts"

If you would like this to be the default runlevel, you can edit /etc/inittab and set the default level to 5 instead of 3:

vi /etc/inittab

Change:
id:3:initdefault:

To:
id:5:initdefault:

Save changes and reboot machine