Is this a new problem? Were things working OK previously? Any new hardware and or software?
If this is a new problem try a System Restore to a point prior to the problem:
start > All Programs > Accessories > System Tools > System Restore
I can find no other precedent for your exact issue but you could try clearing the notification area in an attempt to reset it as seen here:
How to Clear Past Items from the Notification Area
http://support.microsoft.com/kb/283084/en-us
IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs.
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Altering system files; & or modifying the registry can be risky and D-A-L.com and its members cannot accept liability for any adverse effects caused by following advice freely given on this site.