Vista Network and Volume Icons Disappear from the System Tray
It seems to be a common problem that your Network and Volume icons may disappear from the system tray. If this happens, it may be a quick fix by going to the properties of the taskbar and checking off each one on the Notification Area tab. If however, these options are greyed out, then the problem may be the icon “History”. To resolve this, delete the following keys in the registry then restart the Explorer.exe process.
HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion \TrayNotify\IconStreams
HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion \TrayNotify\PastIconsStream