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:
Or, you can create a .reg file which will do the same as above, just automatically:
Windows Registry Editor Version 5.00
Then you should be able to re-enable the icons.
Another option is to use the following batch file (copy and save as "trayiconfix.bat"):
:: Delete iconstreams so next reboot will not have missing/gray icon problems
echo Running tray icon fix...
set ROOTKEY=HKCR\Local Settings\software\microsoft\windows\currentversion\traynotify
set USERKEY=HKCU\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify
:: Delete root streams
reg delete "%ROOTKEY%" /v PastIconsStream /f
reg delete "%ROOTKEY%" /v IconStream /f
:: Delete user streams
reg delete "%USERKEY%" /v PastIconsStream /f
reg delete "%USERKEY%" /v IconStream /f
After running the batch file, reboot, then you should be able to enable the icons.