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 Download.com 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..
The following link on Cnet which is a download for the Faststone Viewer was flagged by my AVG as containing potention virus...or spyware.