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Please note that the problem has been completely solved.
Thus I receive hundreds of emails, detailing ever more different solutions to the problem, each of which has been tried and worked on one computer.
We have to suspect that each of these many solutions works only on a few computers.
The problem is even more prevalent on systems with autologon.
The most frequently affected icons seem to be the speaker icon (sound volume) and the power/energy icon.
In this case the main solutions below will not work. There are a few other problems that can cause similar behavior and can mislead you into believing that they are the same problem.
If you are not sure, read the entire article first, because a few more are described later.
The system tray (in short: systray, nowadays also called taskbar notification area) is the icon strip normally located near the right end of the task bar, next to the clock.
Some Windows XP installations show a peculiar defect in that some systray icons disappear or, rather, do not appear, when the system is booted and the user logs on.
(By the way, don't mistake UPn P for Pn P, Plug-n-Play, which is mostly a different thing.) Microsoft has not taken notice. Some users report successes with various changes that influence the boot and logon timing, but these changes do not reliably solve the problem.
Often they make the problem occur less often though and make it bearable.
An earlier hypothesis, that is loaded too late, has been refuted by Thomas Gregory Platt (see his email below).