Happened to me yesterday. One can never be too careful, but it’s most likely a false positive. If you want to keep playing the game, open up your anti-virus and restore the executable from the virus vault.
If you can’t do that, doing what Flan suggested should be your next step.
If that doesn’t work and you want to download the game from scratch, uninstall it through Steam and make sure to delete the game’s folder before installing it again. The suspect file will still be there after you uninstall, only hidden and marked to be inaccessible, and it will interfere with the new installation unless you delete it somehow first. Deleting the folder takes care of that.
Keep in mind that after installing the game, or after successfully reacquiring the file by verifying game cache, the executable will be marked as a potential threat once again and you’ll need to add an exception for it through your anti-virus software.