I've noticed this on an install of XP, Vista, and 7 on the same machine too. It seems to be hardware-related in my honest opinion. For whatever reason, certain systems I've used tend to have issues loading and will hang doing such for a few minutes before suddenly loading it all at once, including Avast!'s services. It seems to be uncommon, though, and system specs don't appear to play a role in it as the system is pretty powerful. I haven't seen it happen on Windows 8 yet, but my best guess is probably something Microsoft released in it's updates conflicts with certain drivers or hardware.
As bad as it sounds, my personal workaround was to just let the computer sit for 10 minutes or so and do it's thing. When it felt good and ready, it would suddenly load everything and get back on track again.