Your IT dept/Admin must permit and publish WDs definitions from the internal
corporate WSUS server.
So talk/mail him/them and ask for permission.
AutoUpdate--the native automatic update mode of Windows Defender uses the
BITS service. This service is specifically designed, as I understand it to
minimize bandwidth impact.
http://msdn.microsoft.com/library/default.asp?url=/library/en
Description of Windows Defender definition updates
http://support.microsoft.com/default.aspx/kb/915597/en-us
How to troubleshoot Windows Update, Microsoft Update, and Windows Server
Update Services installation issues
http://support.microsoft.com/Default.aspx?kbid=906602
If your windows update is pointing to your local WSUS, then you must include
the Windows Defender in synchronization setting.
In Products selection, please include Windows Defender
In Update classifications, please include Definition Updates
You must synchronize the server and approve the definition for clients to
get the latest definition.