Hello,
If you run under xdm (/usr/X11R6/lib/X11/xdm):
in TakeConsole:
# to kill any xscreensaver that the users launched
xscreensaver-command -exit
in your .xsession:
xscreensaver -mode xxx &
if you run from startx:
in your .xinitrc:
xscreensaver-command -exit
xscreensaver -mode xxx &
(see man xscreensaver)
A good example:
http://www.bodenstab.org/xdm.html