I found the easiest way to deactivate XDM on startup was to simply rename xdm to xdm.d in the dir </etc/init.d>. Now when the computer starts up, it stays in the text mode until a user wishes to enter x by typing <startx>. All you have to do is enter the /etc/init.d directory and type mv xdm <newname>, newname being any name. The computer will not recognise it and xdm will fail to startup on boot. This is the easiest way I figured out how to use the text mode and X interchangeably.
Hope this helps.
