Log on to a console (i.e. not KDE) as root, then enter the command :
dpkg-reconfigure xserver-xorg
which will give you a dialogue to configure your X system. This should offer you a choice of drivers; if you are lucky there may be one for your graphics system, already installed and available to replace the generic driver that was probably installed by the hd installer.
Then log out of the Console; after a few moments the KDM login screen should reappear.