Did you try verifying local files for the game?
Are you shutting down all non-essential programs (especially anti-virus) before starting the game? (in particular, anything that may have been installed or updated since you last tried the beta)
You can try starting the game directly from the ../SteamApps/common/Dragon Commander/Shipping/DCApp.exe file, but there shouldn't be an issue going through Steam (the Steam client is required for the beta, in any case).
Try renaming the graphicsettings.lsx file in the folder below, so (depending on how far into the load process the game crashes) it will auto-detect the graphics settings again.
C:/Users/ %username% /My Documents/Larian Studios/Dragon Commander Beta
Actually, before about a week ago the beta was just using 'Dragon Commander' as the folder, so maybe there was an issue changing over to the 'beta' folder?
If that doesn't help, you can edit the graphicsettings.lsx file in Wordpad, or other text editor, to manually check/change the resolution or display mode:
For the resolution, look for the section
<node id="ConfigEntry">
<attribute id="MapKey" value="ScreenHeight" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="768" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="ScreenWidth" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="1024" 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>
You can email supportdc@larian.com, with a description of the problem and a dxdiag report.