I just scanned the Faststone Viewer with 3 different scanners, and none found it to contain a potential virus. I believe it's a false detection on AVG's part.

I would suggest uploading the file to VirusTotal. It's an online service which analyzes suspicious files. If you find it is virus/malware free, I would then report it to AVG, in order to make them aware of it.

In addition to the AVG Forum, CNET also has a Site Feedback Forum. I doubt you will need to post there. Once you upload the file, I think you'll find it was an erroneous detection.

Best of luck..