maybe it is indeed a virus? I know the official r148 is not infected, but maybe the file you have has been compromised.
Did you download from scirra.com? Is it possible it could have gotten infected somehow? Try submitting the exe to jotti to see if any other anti-viruses report it as infected.
That being said, AVG is really bad (second only to norton anti virus), so it's likely that it's just a false positive.