Well guys, thanks to your help now everything seems to work....
What I did ?
First, I rerun again the Avast cleanup utility. It asks for a directory name, where the program to remove is installed, I left it blank, as that directory did not exist anymore... but the cleaner did not complain of this.
Then I uninstalled completely ZoneAlarm.
Then, following this suggestion
1. Save a copy of newest version of Avast (5.0.677) for the version you need and save it to your HDD:
Pro – http://www.avast.com/pro-antivirus#tab4
I downloaded again Avast 5. BTW, it looks like there is no way to download directly the version 5.0.677
What that link points to, is the installer for version 5.0.594
After having installed it, clicking on "Update", it downloaded the files needed to upgrade to 5.0.677
IMHO, it would be better to have on the Avast site a link to
directly download 5.0.677
Not everybody thinks to check for an upgrade...
Then I restarted the PC, and.. hoooray !... this time no lockups !
Then I reinstalled ZoneAlarm, which did see that I had another antivirus running and asked me if I wanted to disabled its antivirus function, which of course I answered positively.
After another reboot, needed by Zone Alarm, and which was the one I feared more, luckily Windows started without too much fuss... so now I have the firewall of ZoneAlarm running, together with Avast 5 Pro.
Thanks again
Alberto