Have you tried the Verify Files thing on Steam yet?

One rather annoying option would be to uninstall the game (if you haven’t ever got into the game, you wouldn’t lose any progress) and then cleaning up your Windows Registry for any artifacts (you can use free software such as CCleaner or Glary Utilities for this - don’t touch the Windows Registry manually unless you’re absolutely sure you know what you’re doing), then reinstall the game.