Orcs Must Die! won't launch : & ...

I tried the Orcs must Die! demo and it worked great. So I purchased and downloaded Orcs must Die! via Steam. At launch I get the same error every time I try and run it: "Data validation failed. Please validate your game files through Steam."

I have verified the game cache many times, restarted Steam, even uninstalled OMD and re-downloaded five times. Hours later of searching this and Steam forum and trying things, I am very disappointed and frustrated, with the full game never having worked.

Update: I have reinstalled both Steam and Orcs must Die! again with no luck. I have also tried downloading from three different Steam servers worldwide. No Dice.

FYI When I Verify Integrity of Game Cache I get this:

Validating Steam cache files (0 of 1)
Validating Orcs Must Die! file (1 of 1)
>>
100% Complete - All files successfully validated.

Also just tried Orcs Must Die 2! demo. It's working fine.

Reading through other threads on the forum here, I have found people mentioning folders in the OMD! installation that do not exist in my installation. Maybe Steam is not correctly downloading all it should?

Help! The orcs have me almost overrun!