The blue ball icons have disappeared from my system tray too.
I had just upgraded from Windows 98SE to Windows XP Professional.
I right-clicked on the Taskbar and chose Properties and on the dialog box “Taskbar and Start menu properties” unchecked the Hide inactive icons check box but this did not work.
I uninstalled avast! 4 Home edition and downloaded and re-registered but this did not work either.
Resident Protection is disabled but I cannot find a way of enabling it.
Many thanks for your help.