Norton Internet Security System Tray problem
After an update to the latest version of Norton Internet Security (to 188.8.131.52) the icon in the system tray went away, only to return when I specifically started the program. After closing the program, the icon stayed around for a while and then disappeared. Not sure whether this was disabling the program, I did some research and found an entry on the Norton site, here, about an obscure interaction between NIS and TortoiseSVN, a client for SVN repositories. Well, I just happen to be running TortoiseSVN. Following advice, I installed the latest version of TortoiseSVN, and the problem seems to have gone away. At this writing, the latest version of TortoiseSVN is at this URL.