What version of Windows do you have installed? I think the location varies by Windows releases.
On Vista/Win 7, it's in C:\Users\YourName\AppData\Local\Divinity 2\Profile
In that folder is a file called graphicoptions.xml. You might try just renaming the file (or moving it to a different location), in case you need to restore it for some reason. Then, start Divinity II and hopefully it might fall back to default settings.