Installing Avast update has stopped me from using Windows XP

About a month ago I got the message that there was an update available for Avast, I started the update and when it was finished it asked to reboot. When I rebooted all I got was my desktop picture…no icons and no programs bar or anything just the picture. I ran Avira virus recovery and it found and fixed it. Everything ran for about a month and yesterday I was on my laptop and Avast asked to update, which I did, then rebooted and everything was fine. Later in the day I was on my PC (the one that got the virus after the Avast update) and Avast wanted to update on this computer, so I did. When I rebooted I got the same problem that I had last month. Has anyone experienced this ??? Could it be a conflict with the software that I run on the PC??? The PC is running Windows XP and Firefox. My laptop is running Windows 7 and firefox, both use ZoneAlarm .

ZA is the culprit.