The SLiM login manager can be made the preferred login manager of Slackware
in runlevel 4 (graphical login) by editing the file /etc/rc.d/rc.4 -
Use the patch below to add SLiM right at the top before GDM/KDM/XDM.
Run these two commands (use the full path to the rc.4.patch file if you
keep it in another directory than '/etc/rc.d' ):
# cd /etc/rc.d
# cat /full/path/to/rc.4.patch | patch -p2

Eric Hameleers <alien@slackware.com>

-- /etc/rc.d/rc.4.orig 2008-06-23 21:47:17.000000000 +0200
+++ /etc/rc.d/rc.4      2008-06-23 21:46:49.000000000 +0200
@@ -12,6 +12,13 @@
 # Tell the viewers what's going to happen...
 echo "Starting up X11 session manager..."

+# Try to use SLiM login manager.  This comes first because if
+# SLiM is on the machine then the user probably installed it and wants
+# to use it by default:
+if [ -x /usr/bin/slim ]; then
+  exec /usr/bin/slim
+fi
+
 # Try to use GNOME's gdm session manager.  This comes first because if
 # gdm is on the machine then the user probably installed it and wants
 # to use it by default: