Paint this scenario:

[YOUR BOSS]:”What is this evolution crap?, What is this bluetooth service?, What is this pcscd-thing?, What is GAM-SERVER?!" (while referring to a freshly installed CentOS 5.x dedicated mail server ps output).

[YOU]: "No worries Boss, I'll get rid of it!" Then you end up spending hours removing random "unuseful" services.

Now, if you loosely remove too much (as I did...!), after rebooting, you may end-up "welcomed" by an X/twm server in all it’s magnificence...don’t panic!

 

Through the CLI, it is possible to (re)install or restore gnome2, just type (as root) the following at a terminal:

yum -y groupinstall “GNOME Desktop Environment”

The next time you reboot, you’ll get your much-desired gnome 2 desktop environment back.

Catch: if, for gosh-knows-why, Gnome 2 still doesn’t start, check "/etc/sysconfig/desktop" contents, and (if empty), add:

DISPLAYMANAGER=GNOME

From:
https://www.centos.org/modules/newbb/viewtopic.php?topic_id=17751&forum=37&post_id=65444
http://www.linuxquestions.org/questions/red-hat-31/stuck-w-twm-window-manager-in-centos-5-3-a-780599/