Today, I was "playing" with the Linux configuration (as usually) and using tasksel on my Debian I remove the Desktop section, and installed it again, when I tried to use startx I just could not, there was no startx command.
After reading a while I found that I needed to install xinit package to have startx in my machine.
sudo aptitude install xinit
once it is installed you have to create or edit your
$HOME/.xinitrc file, and write in there
something like this:
#!/bin/sh exec startfluxbox #exec gnome-session #exec startkde
be sure to have only one Desktop uncommented.