On a fairly regular basis the 3 right most icons in my System Tray disappear. The Battery, Volume, and Network icons, some or all, disappear.

After doing all the normal things like right clicking the start button and disabling the "hiding" of notification icons, rebooting the system, etc. I still could not resolve the issue. Then I stumbled on a blog post by Colin Cochrane which resolved the issue. Check his post out for detailed info, but the quick and dirty fix is:

Launch RegEdit and go to:

"HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion \TrayNotify"

Delete the values from the IconStreams and PastIconStreams registry keys.

Close Regedit and launch Task Manager, find and kill the explorer.exe process, the select File | New Task from the file menu and enter explorer.exe in the dialog box.

Bingo! All your icons are back!

Thanks Colin!