No idea.
I had avast!5 Pro installed and Windows XP SP3 would eventually load to just the wallpaper- no task bar, no system tray - nothing, then after several minutes more, Windows would load.
I uninstalled Pro and installed it on another computer and experienced no problems. I then installed FREE on the problematic computer and all seemed well but after a couple of days I experienced the same problem.
I suppose the fix COULD have been to reformat the drive and start over but if the problem came back, I would have been sick over it.
I installed another antivirus program and experienced no problems. What hurts is that this all occurred on the computer which I use the most.
I suspect that something on this computer is causing avast! to fail but I have no idea how to identify it.
If I ever have the occasion to reformat, I'll give Pro a try again.
edit: I also had problems with two parts of the scan function (same computer) but I have probably bored you enough.