Start Linux Centos in Text mode

Edit /etc/inittab and look for a line that begin with id:5.

Replace the 5 in that line by 3.

Brief description of runlevels here, but shortly:

Runlevel 0 and 6: halt and reboot the machine, respectively.
Runlevel 1: No services running, only root can login.
Runlevel 2: Users can login but no networking.
Runlevel 3: Networking and text-mode.
Runlevel 4: unused.
Runlevel 5: GUI.