Please look in Documents/paradox/ck2/gfx/flags/ folder. There should be two .txt files and 3 or more .tga files there.
If there are no .tga files there, that is the problem. An antivirus app is getting in the way of the game creating those files. Avast and AVG are famous for this. Avast and AVg are th emost common causes.
Either you need to unload that AV app then run the game so it can make those files, or we have to make them separately. Sometimes users have installed the game on some other computer, let it make those files, and copy them across.
IF that doesn't help, DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/CK2/settings.txt.
From your Documents/Paradox/CK2/logs/ folder, attach system.log .