System Tray Vanished ?

This morning when I turned on my computer I noticed I had no System Tray where it should appear on the right side of the task bar. The digital clock was the only thing showing. There are no icons where there are usually about 6 or 7, and the expansion arrows are also missing. I wanted to be sure I had no virus and it checked out clean with both Norton and McAfee. The computer seems to function properly, at least for now, as I can surf and check mail without a problem. I've done all the local maintenance and rebooted several times but the System Tray is gone!

 

I have no idea where to check now to bring it back and hope it is not a registry problem. Wonder if there is a possible cure other than to reinstall XP. Any help is appreciated. Many thanks.