As a IT Admin you cannot customization the information that is popup as balloon and also the information shown in the "Network access protection" window. But you can still provide additional information which is more useful specific to your organization by providing customized title, description and more at More Information. Here is the link on how to configure those
http://technet.microsoft.com/en-us/library/dd314157(WS.10).aspx.
Thanks
-RamaSubbu SK
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