Well I've really done it now. Here's the story: I ran out of space on the Mandrake 10.1 partition and I couldn't log-in, so I used a Knoppix CD to delete some files that were backed up on a slave hard-drive. That's the short version. When I logged in I wasn't able to click on the desktop KDE icons and get them to open. The start hourglass would tumble in the panel but would disappear after a long try and . . . nothing. I could however click on most of the links in the KDE panel. Then I noticed that in the terminal what I think is my "host name" has been changed to "@Knoppix." Also, I wasn't able to link to Konqueror through Kmail messages, that got me into my home .KDE folder where I saw that there were two sets of sub-folders half named after Knoppix and the other half w/the older default name. I think what I need to do is edit my user name (?). But, frankly I don't have a clue. Any ideas would be greatly appreciated. Thanks in advance,

M

I had a similar problem just the other day
I have a multi-boot system with two Windows XP installations, Mandrake LE 2005 and had just did a fresh Fedora Core 6 installation. No matter how many Linux distros I install, all will use the same Home partition, normally I would create a different user name for every distro and there will be a folder for every user in the Home directory. Anyway, for some stupid reason I though I would try to use the same user account as my Mandrake this time thinking it may work as they are both Red Hat derivatives, I use only KDE in Mandrake and when I logged into KDE from Fedora it asked me if I wanted to use KDExxx as my default window manager (the xxx's represent a number I forgot as of this writing) a split second after clicking yes, I realized that was the default KDE from Mandrake, and got kicked back to the log in screen where I logged in as root to get back into Fedora. Then when I later booted Mandrake all I got was the original splash that comes up before KDE and that's as far as it went, it was frozen there. I originally set up Mandrake to automatically log into my user account whose KDE settings were all screwed up by Fedora 6 which has KDE 3.5.4 and Mandrake has KDE 3.0 (I think), two different versions of KDE had altered desktop settings and neither could start that account.
You have a similar situation but better than mine in that I only had one desktop directory with settings from both, you apparently have two desktop directories, one from Mandriva and one from Knoppix.
Just a suggestion here:
If you can log into a terminal (maybe failsafe selection from menu before kernel gets loaded at bootup) and delete the Knoppix KDE directory in your home folder, and use "hostname" command to revert back to username@localhost replacing "username" with your actual user name, type $man hostname if you need to figure out how to use that command as I've never used it.(After re-reading your post it appears you have the same situation I had but does'nt hurt to try)
If not you can also try the "useradd" command as root (type $man useradd for more info) to make a new user and transfer all data from original user account to new one, and log into graphical environment under new user, and if you prefer using original user name just delete it as the desktop settings are no good and re-create another account with that preferred name and transfer all data back and delete un-wanted new user. You will be back to default desktop settings and will have to re-do your color scheme and the likes. If you originally saved your color scheme in KDE control center you can just select it and that part is taken care of. I used the useradd command about three years back and all I remember is that it was fairly straight forward and easy.

Thanks Junior Hacker those are all interesting ideas. I think the safest thing to do would be to add a new user and transfer data. Though burning the whole house down and re-installing has also crossed my mind

Nothing wrong with that, as I tend to advertise at the bottom of my posts.

I use a boot manager called bootitng which can be acquired at bootitng.com and used for free. I make a 10GB partition and put a fresh install, get all the updates and use bootitng to make a compressed image of the entire partition and store it on my data drive. Then I expand the partition if need be with bootitng and go into the linux operating system and issue one command "#resize2fs /dev/sda1 in my case to have linux expand it's file system to take up the full allocated partition. Reason I do this is because if I screw something up during installation (which happens often enough because compiling pci modem drivers usually are troublesome) I wipe that 10GB partition and re-load the image in less than ten minutes with a few clicks of the mouse, no sitting around plugging disks into the drives for an hour + to start over. This method is also nice in situations like we both experienced lately. I do have a DELL XPS powerhouse which helps me do this in less than 10 minutes.
Just a little tip.

yep, a new user is the quick fix, a complete reinstall is the correct one...

Thanks all. Finally got a free chunk of time to re-install. That did the trick.