I don't recall anyone else describing this issue.
Did you change the resolution? Trying a lower resolution and switching back might reset whatever the problem is, or a different resolution may work properly.
In Windows Explorer, try browsing to the folder below, open the 'graphicoptions.xml' file in Notepad (or other text editor) and make sure the proper resolution is listed. You could also delete the file, so the game would recreate it the next startup (though you would have to re-do your setting tweaks).
XP
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Profile
Vista/7
C:\Users\ %username% \AppData\Local\Divinity 2\Profile
