I have here a Hercules eCAFÉ EC-900CW from Guillemot that has WinXP preinstalled. Standard-Virus-Protection is Avast - but I have on other PCs also Avast. Not only that the eCAFÉ is brand new - I just have made a total reset with new installation of the whole WinXP to observe the Problem.
As the user insists of having his own system appearance colors, the display settings are set to "Windows classical" - color scheme "Plum (high res.) - with several colors adapted manually.
Configuring the system worked fine - and the system tray always had about 8 to 10 tray icons visible. But as soon as Avast was installed, the system tray only shows the icons "wireless", "wifi" and "avast" when starting up the system. Making a log-off and a re-log-on, many more icons are there - as they also should directly at system boot: "LAN", "wifi", "realtek audio", "wireless", "audio/loudspeaker", "screen toys", "battery/power", "avast". Please note that ALL of them have the "show icon in system tray" selected!
This system is not the only one that the tray icons are eaten away - also on other systems the same thing happens and tray icons disappear. There is an issue known when using "own colors" in the appearance - but the question is: what is involved? Avast seems to be one of these that actively don't let icons appear at system boot. And it is also not obvious why the icons don't appear directly at system boot, but they appear after logoff and relogon.
Thanks for finding solutions to that issue.