Just wanted to add this to Kenneth's reply. Perhaps you should try:
startx /usr/bin/startkde
or
move ~/.xinitrc (if it exist) to something else, and:
echo "exec startkde" > ~/.xinitrc
startx
However, if the same situation arise with kde/gnome/others, then its probably due to your xfree86 settings or graphics card. Did you happen to change anything with X display config?