Try starting the game directly from the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file, by right clicking and selecting run as administrator.
Try manually setting the resolution, and running in windowed or fake fullscreen mode. If the game doesn't get far enough to create the configuration file mentioned below, try temporarily lowering your desktop resolution, or disabling Eyefinity.
You can edit the graphicSettings.lsx file (in the '..\Documents\Larian Studios\Divinity Original Sin' folder) in Wordpad, or other text editor, to manually change the resolution. Search for the section:
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="
ScreenHeight" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
1600" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="
ScreenWidth" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
2560" type="4" />
</node>
Search for 'fake' near the start of the file to set the display mode (if both FakeFullscreen and Fullscreen are set to zero, it will be in Window mode).
<node id="ConfigEntry">
<attribute id="MapKey" value="
FakeFullscreenEnabled" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value"
value="0" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="FrameCapFPS" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="60" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="
Fullscreen" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value"
value="1" type="4" />
</node>
