Hello,
I just registered to tell of the same problem.
Avast Home Edition was running fine for a long time on my XP PRO SP3, yesterday I got the little green box telling me of a newer version, but I ignored it.
Today my computer starts up normal, until logon screen, after logon it just shows the background of my user but nothing else happens.
I tried several reboots and once I got the task manager up and could see a process called "avast.setup".
I could not kill it for some unknown reason.
When I ran safe mode of XP I used MSCONFIG to uncheck the avast services and after that the machine boots just fine, but avast is disabled.
As soon as i activate the avast services and reboot it will hang up again after logon.
So far so good... I then retried complete uninstall of Avast, then reboot, then the uninstall tool, reboot again, then fresh install of avast.
First reboot then went well, everything was back to normal. Then I just switched the sounds of avast off, and tried another reboot to be sure everythings working again.
And know what? It hangs up again after logon.
Now I disabled the avast services once again in safe mode to be able to use my computer at all.
Greetings