Avast is my main Antivirus detector too, and it hasn't detected anything thus far (although I only downloaded the Zip). I think it's a false positive to be honest. (And having played for a few hours, also seeing the file structure of the game, e.g, A few standard Microsoft dlls and a data directory with just the expected .pcx, .wav and .ogg extensions, I think this is as clean a package as you'll get!).
Also, it's a great game :)