To fix open up your killingfloor.ini file ( by default located in steam/steamapps/common/killingfloor/system)
Find the engine.engine section and the following lines:
and change them to
This will change to game to DirectX8 rendering.
You will also want to change your resolution to your desired. In the same ini file look for the following lines in the WinDrv.WindowsClientsection :
And change the Full Screen and windowed to your desired resolution for full screen and windowed mode. To toggle the game to launch in windowed mode, fine the line FullScreen=True and change it to False.
Save the file and launch Killing Floor again.