That's not something that can be fixed in Slackware, it's something that should be fixed in VirtualBox.
I install slackware 14 rc3 in virtualbox from opensourcelab rsync mirror.
Option "Composite" "Disable"
I doubt this is sufficient. The problem is that kde on startup checks the capabilities of the graphics driver in use. That check somehow crashes kwin, so you have to disable the check.
You can do this by putting:
into ~/.kde/share/config/kwinrc for every user or globally into /usr/share/config/kwinrc