This is a problem that people have reported previously and it seems like this one is a bug in Windows XP. So far Microsoft has acknowledged this problem but hasn't offered yet a solution of fix since it doesn't seem "reproducable". The problem I am referring to are the missing icons in the Taskbar Notifcation Area. Like the volume control icon that is missing or the network connection icon and so on.
Since the cause is not known for sure there are several speculations of which the most plausibel is that the taskbar notifcation area service is not ready at the time several applications are started. Other people say that this is caused by the Universal Plug and Plug services.
There are several workarounds floating around the internet but there is one workaround on my workstation that did the trick for me and it involves changing the startup type of two services to "Automatic". The services I am talking about are the SSDP Discovery Service and the Universl Plug and Play Device Host.
If this doesn't work for you try to do a Google search on "Missing Taskbar icons" and try the different proposed solutions.
Cross-posted from
The .NET Aficionado