If icons have disappeared from the Taskbar's notification area, there are two things to try: first, press the Windows key and [R], type "regedit" and press [Enter]. 

Browse to HKEY_CURRENT_USER\ Software\Classes\Local Settings\Software\Microsoft\Windows\ CurrentVersion\TrayNotify, and delete both IconStreams and PastIconsStream values.

Reboot, or log off and back on again. If the problem persists in XP, and you can live without it, open the Add or Remove Programs Control Panel, click Add/Remove Windows Components, expand Networking Services, and untick "UPnP User Interface". Then click OK > Next.