System icons do not appear in the notification area in Windows Vista until you restart the computer

1. Click Start
   
   , type regedit in the Start Search box or the Serach programs and files box, and then press ENTER.
   
   
   
   If you are prompted for an administrator password or for confirmation, type the password, or clickAllow.
2. Locate and then click the following registry subkey:
   HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\TrayNotify
3. In the Details pane, click the IconStreams registry entry.
4. On the Edit menu, click Delete, and then click Yes.
5. In the Details pane, click the PastIconsStream registry entry.
6. On the Edit menu, click Delete, and then click Yes.
7. Exit Registry Editor.
8. Restart the Explorer.exe process. To do these, follow these steps:
1. Press CTRL+SHIFT+ESC.
2. On the Processes tab in Task Manager, click the explorer.exe process, and then click End Process two times.
3. On the File menu, click New Tasks (Run), type explorer, and then click OK.
4. Exit Task Manager

http://support.microsoft.com/kb/945011