I have had experiences with several of these downloader.something.something viruses, and what has always worked for me was AVG. It's a totally free antivirus program. Once you install it, it stays running in your system tray, and it will scan outgoing and incoming email, scan your system, and automatically update itself, if you tell it to.
Anyways, when I had those viruses, it would tell me that it found it on my system. Then I had to run full system check, and it found the viruses, and removed them for me. If it couldn't remove them, it would give me the choice of deleting the corrupted file.
Google AVG. It's free, and just as good as Norton, IMO.