If you downloaded it from the site, lots of users can confirm that it is not, in fact, a virus. Can you tell your anti-virus software to ignore it or something? I promise it's safe.
.exe files are typically registered as a virus by antivirus programs. It's only cautionary, if it prevents you from opening up the program, make sure you allow it through, and if needed, shut off Sandbox mode.
I always have that problem too and there's no way to make Avast ignore it actually. So the only thing you can do is to temporarily desactivate it everytime you want to open SMBX. I'd love to have a better solution to this though.
A: Use a better antivirus
B: I would suspect the culprit is lunadll, because it's a dll that injects code into the base exe. Lot of antiviruses go crazy over things like that.