Yeah, it's in the downloads window. After the download completes, a bunch of little wavy lines (graphics) run across it, and it says "scanning for viruses". I've now disabled that in my about:config for Firefox 3.6.16, and hopefully won't see it again. I'll report back here if I do.
I've been monitoring my avast processes, with all of the avast Shields disabled, and don't see any activity there during the download process itself. Even when enabled they weren't sucking off much processor time, but something was, and I thought it could be related. That other something is my ZoneAlarm 6.1.744.001 vsmon.exe process which is sucking off 15% or so of processor time during downloads. It seemed to be worse if I'm running avast at the same time, which is why I turned avast off. But I can't seem to get away from it... I tried putting the IP of the download website into my ZA trusted zone, but that didn't do anything. I also tried turning off ZoneAlarm and substituted the windows firewall, but then the same thing happens with my System process. So maybe I just have an old, slowpoke XP system on a 7 year old computer...no cure possible(?)...but I need to milk it for whatever it's worth.