as root from the console, the command "service dm restart" should get X going again without doing a startx.
Beyond that, what do you mean that Xserver and KDE "fail"? What error messages are there? These might appear in a window on the screen, or in the console window, or in the file /var/log/messages, or not at all.
What version of KDE are you running; what version of Xorg? I ask this because there have been some updates since 2007.1 came out; are you up to date?
What video card? What video card driver? Have you had any abnormal events with your system that might have corrupted a library (lose power while writing to disk, for instance)?