Sometimes the battery, volume, and network icons disappear from the system tray and you can't re-enable them because they are grayed-out. This can be caused if Vista crashes while shutting down for a reboot, but also can be caused by other things (who knows?).

There are a bunch of ways you can fix this, you only need to pick one:

Manually editing the registry

1. Open regedit as Administrator
2. Search for all instances of "IconStreams"
3. When found, delete "IconStreams" and "PastIconStreams"
4. Keep searching until you reach the end of the registry
5. Reboot
6. Go into the Start Menu properties an re-enable the icons

Load the fix as a .reg file

Or, you can create a .reg file which will do the same as above, just automatically:

1. Open "Notepad" or another text editor
2. Copy & paste this code and save as file name "trayiconfix.reg":
   
   Windows Registry Editor Version 5.00

   [HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify]
   "PastIconsStream"=-
   "IconStreams"=-

   [HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify]
"PastIconsStream"=-
"IconStreams"=-

3. Double-click on the file to enter it into the registry
4. Reboot the system
5. Go into the Start Menu properties an re-enable the icons

Then you should be able to re-enable the icons.

Use a batch file

Another option is to use the following batch file (copy and save as "trayiconfix.bat"):

After running the batch file, reboot, then you should be able to enable the icons.