Don't know how accurate this is, but presumably some ad on the
forum main menu page upset my Avast, which announced that it had
"blocked a malware URL", which it then gave as
http://cookiex.ngd.yahoo.com/v2/cexposer/SIG , "Infection:URL:Mal".
I have no idea if this information is of any use, but I thought I
ought to mention it. The Avast page went on to say that although
the site may be legitimate, "The latest research from the avast!
Virus Lab shows that more than 80% of malware (viruses, spyware,
and the like) spreads through legitimate websites"...

I use Avast and have never had this, But I do know that Avast is notorious for giving false positive
virus and trojan warnings.

I've just had a threat blocked message using AVG as well whilst viewing the main index page.

Just had the same issue with Avast on Win8 running Firefox for the first time. Went and clicked on the avast
maleware blocker box to see what the issue was and it took me to Avast central and tried to sell me the advanced
version. I use the free. Now they are resorting to tricks to get you to buy.