I am using Avast Home 4.x. I am trying to download the file b5900mux.exe, a printer driver from the Canon Europe web site. Each time I try to download Avast tells me this file is infected with a trojan. Canon basically say that this is laughable and that this file has been downloaded over 20,000 times.
Check this topic out http://forum.avast.com/index.php?topic=23811.msg196356#msg196356 whilst it doesn’t not look the same it could well be related, especially if the virus name is the same, which you don’t mention ?
You should also ensure that you have the latest VPS update, 0639-4 (right click the avast icon, about avast!).
You could pause web shield that should at least let you download it, standard shield might have a pop at it too. So long as you don’t try to execute it before you investigate it.