Just re-installed Visual Studio 2008 SP1 on my brand new Windows 7 RC, along with SQL Server 2005. I was struggling with the following error for a while, since I wanted to create a Database Diagram.

TITLE: Microsoft SQL Server Management Studio
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Database diagram support objects cannot be installed because this database does not have a valid owner.  To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.
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BUTTONS: OK
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Here is what resolved it for me.

    Right Click on your database, choose properties
    Goto the Files Page
    Enter "sa" in the owner textbox.
    4. Hit OK

Hope this helps,
Rahul

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