Try a full clean reinstall:
If this is Steam: uninstall the game from Steam and move any valued save games elsewhere. Manually delete the CK2 directories from under the Steam AND MyDocuments folders, now reinstall the game via Steam.
If GamersGate: Move valued save games elsewhere, and uninstall the game. Manually delete the CK2 directories from where GG put them and from MyDocuments. Reinstall the game , download 1.05b patch again and apply it.
If that doesn't help, please post a dxdiag.txt (you can delete everything after the "Sound devices" section for brevity)