Not all browsers have the avast Webshield which is the scanner that, I suspect, is finding the problem with this download. Perhaps you can advise us of the av products that you have used previously that do allow you to override an http scanner. Even so, I suspect that the majority of us are willing to live with this "problem" of an occasional false positive rather than download an infected file.
I understand that a false positive is irritating but you not need to turn off all on access protection. For myself I find that ignoring the alarm from the Webshield allows the download to complete (after a time) without any further problems (unless like me you have a an automatic scan of downloaded fiiles which again reported the file as infected and requires me to tell avast to ignore the suspected infection).
You also have the option to pause the Webshield and download again, you have the option to tell the Webshield not to scan the URL from which you are making this download.
If this has happened on a "number of occasions with legitimate files" (though a scan fails to find any reported in the couple of months you have been registered) then I regret that you must be more unfortunate than most of us.
