If you uninstall the game, reboot, make sure there are no files remaining in the game folder, or configuration files in the settings folder, and reinstall the game, you should at least get back to being able to play at the default resolution.
In XP the saves are in the folder below; you can take out the entire Divinity 2 folder if you don't have any saves to keep (the install program should ask if you want to keep your settings and saved games, but it is better to double check that everything was removed if you are having a problem).
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders.
