You need to have a desktop manager selected in rc.conf. Su to root and edit "/etc/rc.conf" You should find two important entries. One is your boot runlevel, set that to 5, and the other is your X Display Manager, that you should set to whatever you have installed, either XDM or KDM. If Slackware is at all cool, they should have rc.conf heavily commented.
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Note the difference between your display manager and window manager. They are NOT the same.
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The way X Windows works is that at boot, init starts XFree86 and then switches to runlevel 5 (maybe not in that order, but i dont care...). Then Xfree86 launches the Display Manager selected in rc.conf. The DM loads the login screen. There you choose your "session type" or your Window manager, that would be XWM, KDE, Gnome, etc. Then the DM loads the WM and tada, you have a graphical interface.