As I already said: I redownloaded the complete game already Just try it, the worst that happens is that it still doesn't work. First, try the corresponding Steam link for your copy of the game. Validate Multiwinia Validate Multiwinia Demo If the links fail for you, try running a manual verification...

Had this same issue. Having Steam perform a verification brought up one or more bad files, and Steam re-downloaded them, and everything works now.