The process of downloading the website files and scanning the archive with ESET has been advised to me by other web techs! I have successfully used this process in the past to find & delete viruses on other websites!
Thank you for the reports, but I am confused because I see a number of vulnerabilities & weaknesses, not virus infections. Avast's site blocking message is actually headed by the phrase "Infection Detected" in big red letters — is that triggered by vulnerabilities & not necessarily a virus? Anyway, I see several weaknesses & will address them in the order of most severe to least severe. Is there any way of knowing specifically which one triggers Avast's "Infection Detected" block page?
How I was supposed to know that my first support ticket was working its way up the queue when fully one year elapsed between my first and my second submissions without any response?
Unfortunately, I'm unable at present to perform any maintenance that would involve changes to the domain settings, and this would include changing to a host with non-outdated software! The client originally registered the website with Melbourne IT via Yahoo in the early 00s and through the subsequent series of web developers who've worked on the site, the original Yahoo credentials were lost and the business account closed. Apparently, Melbourne IT directs people with this issue to Yahoo, who've since closed any customer support numbers (including those on the whois) and doesn't respond to requests to restore domain access, even if you have access to the owner of the e-mail address and telephone number listed on the site's whois page and can easily prove ownership. The annual billing continues unabated, and so we're still able to keep the domain active but that's it at the moment, as the website can only be edited through the previously configured hosting provider. There are other problems with the host and I would love to change hosts to somebody with better software.
Others with the same Yahoo domains issue:
http://www.webhostingtalk.com/showthread.php?t=919699http://www.webhostingtalk.com/showthread.php?t=864671 (currently attempting the fix which the OP found)