WU Icon appears and quickly disappears when update is available

Hello:

I have Windows Update set to notify me of updates, and ask me when I wish to
download/install them.

Today, April 18 , and the last update on April 11, the Windows Update icon
appeared transiently in my System Tray and disappeared. Same happened when I
tried resetting it to download and install automatically. I recall that the
March updates ran according to the settings. I was notified , ok'd the
download and it went smoothly.

What is causing this issue, and how might I correct it?

Windows XP Pro SP2
Zone Alarm Security Suite 6.1.744.001

TIA

-Eli