In MyDocuments CK2 folder delete /gfx/ and settings.txt and let the game recreate them at default settings. If that doesn't help please post a dxdiag.txt (you can delete everything after the "Sound devices" section for brevity), settings.txt, system.log and error.log files (you'll need to rename the .log files to .txt to upload). Also open setup.log, search for "checksum", and paste that line only in here.