Chuck,
That message is related to inserting a "Like" button and a Facebook link that will display near the top of the page. I first noticed this was added a couple of weeks ago. I subsequently ran two anti-malware scans (Spybot Search and Destroy; Spyware Terminator) on my local system, and found nothing amiss. I believe that you can safely accept the file, and those little buttons will appear on the page (on the "Thread" line). These are just added doo-dahs to the default VBulletin forum application.
Whether or not the warning message appears depends on your local browser's security settings.
Bob