Got a ThinkPad and decided to give a full featured and fancy KDE 4 a try. Very nice deskop environment, indeed. However every now and then, the computer would just lock up. Freeze. Nothing but resetting it from the power button would help. Googled around and found a useful post by AlienBob. Also, reading the official Slackware 14.0 release CHANGES_AND_HINTS.TXT explained how to disable compositing.
~/.kde/share/config/kwinrcand add the following in the
/etc/X11/xorg.conf.d/disable-composite.conf, containing this:
Option "Composite" "Disable"