it appeared on the picture only thread page 370, so it could just be an image from the site link to page: http://forum.grasscity.com/artists-corner/688047-picture-only-thread-370.html
It's just a notice nothing to worry about cause you haven't opened the virus. If you want free antivirus go to pspiso.com, register and go to pc download section
Or just go do a Google search for download.com avg then add malwarebytes too. Don't forget to uninstall Norton before installing any new anti-virus.