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.
XP
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Profile
Vista/7
C:\Users\ %username% \AppData\Local\Divinity 2\Profile
Try running the game as administrator, and (in Vista/7) disable desktop composition in the compatibility options of the program/shortcut.
[SOLVED] Interface Resolution and Game Resolution