My site, moonvalleyna.org, experienced a similar problem with this error message. One user, who was using AVG, suddenly starting getting that error message. We discovered that header.php in our theme (Twenty Ten) had some extraneous code inserted into it. It was HTML labeled as “HiddenDiv” and was attempting to display messages regarding Viagra and Cialis. Have no idea how it got there. We had never updated the theme since its original installation and the administration is limited to just two of us. I updated to the latest Twenty Ten and that got rid of the junk code. Also changed the admin password just in case.
