Missing Icon in taskbar?

Have you ever noticed that small icons in the taskbar(next to the clock). It shows some program that's running and many of the program is launched automatically during start up.
For example, antivirus, winamp agent, sound,local area connection and etc. Some time, or most of the time, one or two of the taskbar icon disappeared.Have anybody faced this situation?
The program is still running, but yet the icon is not in the taskbar. This seems like a bug that's annoying for me. Especially if the missing icon is the antivirus or sounds. I usually use the icon as a shorcut to enable/disable the antivirus or adjusting the master volume.

There is a bug that caused this.We can fix this easily by:

1. Go to Control Panel >Add Remove Programs>Add/Remove Windows Component.
2. Go to Networking Services and click Details.
3. in the Details window, uncheck the "UpnP user interface"
4. close all windows and reboot.
5. Voila! no more missing taskbar icons. :)