System icons may not appear in the notification area on a Windows Vista-based computer until you restart the computer
When you first start a Windows Vista-based computer, one or more of the Network, Volume, and Power Options icons may not appear in the notification area. These system icons do not appear until Windows Vista has completed all its startup activities and processes, and you have restarted the computer. Additionally, the relevant system icon check boxes may not be enabled in the Notification Area tab in Taskbar and Start Menu Properties. However, the Network, Volume, and Power Options features are functioning correctly even if the system icons do not appear in the notification area. You can use Control Panel to access these features.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB945011.