I must note that I had a lot of trouble mixing ZoneAlarm with Avast 5 on Vista Home Premium 32-bit. ZoneAlarm, it seems, is incapable of holding onto an updated version number for a main .exe that is part of Avast. That is, ZoneAlarm keeps reverting to a version number for that file that points it at something that's been replaced. At least that's what I understand about the conflict, notes on which are fairly easy to google.
After weeks of slowdowns and inexplicable sluggish symptoms I yanked ZoneAlarm in favor of Agnitum Outpost Firewall 2009 version 6.5.1 - it's been 100% wonderful (and free).
Avast 5 continues, by the way, to astound me by being the very best antivirus I have ever seen.