Update that nVidia driver:
http://www.nvidia.com/Download/index.aspx?lang=en-us
If that doesn't help, in Windows Explorer click on Documents in the left column. Is there a folder Paradox Interactive there? If so there should be a folder for this game in it, and settings.txt and system.log are there.
If there is not a folder for this game, then it has been deleted since you last ran the game. The game ought to recreate it, if it has noth that can only be one of two thigs: an antivurus app getting in the way of creating it, or the Windows User folder has been moved and the game cannot find it.
What antivirus app do you use? Add ck2game.exe to its exceptions list/Whitelist.
One more possibility that can cause the failure to create the Documents/Paradox Interactive/CK2/ folder, is if the Documents folder has previously been moved so the game can't find it. Is that possible?
Use Notepad to make a new text file called userdir.txt in the Steam CK2 folder where ck2game.exe is. In that file place a single dot, save and exit. This tells the game to write the user files right there instead of under Documents. If that works and the game now runs, you can replace that dot with any file path you like.