I would suggest you start your linux in command line or just press Ctrl-Alt-F5 to start a command line, login as root and type in kde, if it is not in the PATH find out where it is... my is in "/usr/X11R6/bin/kde" once you try to start KDE please get all the info and error out that will get generated and post it here at the forum so that we know what we are dealing with!
Does it crash on any user?
Have you got enough Disk spane on your home/directory?
Good luck with the collection of the info!