XP is notorious for not showing system tray icons properly. If too many things that display icons are trying to load too fast or too simultaneously, many times, some will not show at all. When I was using Comodo on XP, it’s icon would never show until I clicked on the desktop icon to open the GUI. Then the system tray one would show up. One thing you could try is to set Avast to load only after all other system services. This is found in the troubleshooting settings. Also, make sure the box to show the icon is checked in the basic settings. The real solution I found for the same problem using Comodo was to install a little utility called StartupDelayer. You can set how programs load at boot. I delayed Comodo by 10 seconds and that fixed the problem.