Database Diagram in VS 2005 for SQL Express 2005

 I have installed VS 2005 (Beta 2.050215-4400) and SQL Server 2005 CTP (File Version 9.00.1187.07).

I can connect to the SQLEXPRESS database and created tables from the VS 2005. But when I try to click the + to expand the database diagram folder I get the following window:

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Microsoft Visual Studio
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This server does not have one or more of the database objects required to use database diagramming.  Do you wish to create them
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Yes   No   Help  
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After I select yes, I get the following error  message:
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Microsoft Visual Studio
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Could not obtain information about windows NT group/user 'xxxx/xxxx', error code 0x5.
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OK
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The + next to the database diagrams folder disappears. I select the "Add new diagram" menu item from the database diagram folder. I able to add tables to the database diagram. But when  I try to save the database diagram, I get the following error message:

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Microsoft Visual Studio
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(Exception from HRESULT: 0x80131904)
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OK
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Does any one know why