Ok, keeping Steam Cloud off, nuke the game:
http://sd.stardock.com/brad.ather/Jingimages/2015-05-22_1515.png
Let me know if anything is still here:
C:\Program Files (x86)\Steam\SteamApps\common\Galactic Civilizations III
If so, zip that up, then delete that folder.
Make sure there is no MyGames\Galciv3 folder, only the GalCiv3Old you renamed.
Reinstall the game and try again.
If that fails, I want you too look at this page as it has other things that would be rudundant here (video drivers for example):
https://esupport.stardock.com/index.php?/default_import/Knowledgebase/Article/View/496/161/general-galactic-civilizations-iii-troubleshooting
If it still fails, we will need the tool run; use the link to submit it please.