I don't recommend using the Xdisplay properties in the Control Center. It always borked things pretty good for me. There are a few things you can do. The easiest is to boot the Knoppix cd and copy the XF86Config-4 file it creates to your hdd install. It usually works quite well. Or you could copy the info from the Knoppix cd XF86Config-4 file and compare it to the one on your hdd install. Here's a post on how to do that. If you can't get that to work post some hardware details and your /etc/X11/XF86Config-4 file please.