You need to make sure your /etc/X11/xinit/xinitrc points to the correct xinit. For example if you wanted to boot KDE then it would point to xinitrc.kde. That should allow all users of the system to boot KDE. Now if you would rather specifiy for a single user then copy /etc/X11/xinit/xinitrc.kde to your users /home dir then rename it to .xinitrc. That will start KDE for that user only.