UNA Anti-virus? i would have to say i've never heard of it, however I can assure you if there was any virus in it then the "Pie" fools would have posted it all over the place by now, they would have a field day if something like that where to happen, and you would certainly hear about it extremely quickly, lol
however by googling "TrojanDownloader.Win32.IstBar" i get:
Downloader : A program designed to retrieve and install additional files, when run. Most will be configured to retrieve from a designated web or FTP site.
is it coming up with it when winmx is run? if so it could incorrectly be detecting the block list download, although that would seem to be a rather flawed antivirus implementation if it just assumes every file download is a trojan