To restore system icons, including those for Power, Network and Volume, to the taskbar in Windows 7, click Start in Windows, enter "icons" in the Search text box, and click the icon for Notification Area. Locate the icons you want, and select On in the Behaviors selection list.
If the notification settings of the system functions are on but they still don't appear in the system taskbar, delete two subkeys from the registry to make them reappear. Click Start, and enter "regedit" in the Start Search text box. In the Registry Editor, locate a subkey for HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify. Click the subkey, click IconStreams in the Details pane, and select Delete from the Edit menu to delete this entry. Repeat the process for the PastIconsStream entry.