Defender running but not in taskbar, not popping up

[Bert Hyman]

XP Pro SP3
Defender 1.1.1593.0

On reboot, the Defender icon is not in the taskbar, but the app is
apparently running, with MSACui and MsMpEng active.

More important, I did not get an expected popup warning me when an app I
regularly run made a change to the ....\RunOnce registry key.

When I "ran" Defender from the Start menu, the registry change warning was
in the list of problems to be resolved.

Oddly, when I did a "Switch user" to my wife's login, the Defender icon was
in her taskbar, but when I logged her off and switched back to me, the icon
was still missing.

The startup registry key for Defender is

"C:\Program Files\Windows Defender\MSASCui.exe" -hide

which I gather is what it's supposed to be.

This behavior is very consistent with Defender and I don't think it can be
explained away by any general "quirky" behavior of the taskbar.

 

[Bert Hyman]

In Bert Hyman

On reboot, the Defender icon is not in the taskbar, but the app is
apparently running, with MSACui and MsMpEng active.

BTW, when it's in this state, re-running the WindowsDefender.msi
installation and selecting "Update" causes the icon to pop up when the
update is done.

Something else I tried earlier that didn't help was to go into
Tools->Options->Real-time protection options and change "Choose when the
.... icon appears ..." back and forth between "Always" and "Only if ...",
saving each time. Nothing happened.