If you're seeing this error: "Failed to start game(missing executable)" then most likely your anti-virus is removing the games .exe file which is needed to run the game.
From this Steam page you may find a solution to the issue: https://support.steampowered.com/kb_article.php?ref=8676-EFLX-7453&l=english
Where to look for further assistance: http://screencast.com/t/9eJrdvo4B
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