In /etc/skel/.kde/Autostart/ there is a file called
showindex.desktop

and in that file is what makes the webpage come up :
Code:
Name=KNOPPIX 
Exec=kfmclient openProfile webbrowsing /cdrom/KNOPPIX/index_en.html 
Type=Application 
Icon=html 
Terminal=0
you you can see that it loads /cdrom/KNOPPIX/index_en.html

So simply, change the showindex.desktop to a page of your liking.