Don't know what version of Knoppix you are using, but here there is no /etc/skel/.kde/Autostart/showindex.desktop.

This file is generated in /etc/X11/Xsession.d/45xsession, it contains this:

Code:
if [ -e "$INDEXFILE" ]; then
cat >> $HOME/Desktop/KNOPPIX.desktop <<EOF
[Desktop Entry]
Name=KNOPPIX
Exec=kfmclient openProfile webbrowsing $INDEXFILE
Type=Application
Icon=html
Terminal=0
EOF
ln $HOME/Desktop/KNOPPIX.desktop $HOME/.kde/Autostart/showindex.desktop
fi