Database Diagram Support Objects cannot be Installed ... no valid owner

You should consider SQL authentication account for database ownership; then you don't have to worry about accounts coming and going, databases or instances moving to different servers, and your next PC name change. I have several systems where we use:

ALTER AUTHORIZATION ON DATABASE::Ariha TO [sa];

Or if you want to change the owner to that local Administrator account, then it should be:

ALTER AUTHORIZATION ON DATABASE::Ariha TO [DevPC\Administrator];

Because renaming the machine to DevPC has eliminated the local account that used to be named WIN-ND...\Administrator and this has also invalidated the current owner of the database.

If SELECT @@SERVERNAME; is not accurate (it should say DevPC), then in order to ensure that your server rename has taken hold within SQL Server, you may also want to issue the following:

EXEC sys.sp_dropserver @server = N'old server name';
GO
EXEC sys.sp_addserver @server = N'DevPC', @local = N'local';
GO

In SQL Server Management Studio do the following:

1. Right Click on your database, choose properties
2. Go to the Options Page
3. In the Drop down at right labeled "Compatibility Level" choose "SQL Server 2005(90)" 3-1. choose "SQL Server 2008" if you receive a comparability error.
4. Go to the Files Page
5. Enter "sa" in the owner textbox. 5-1 or click on the ellipses(...) and choose a rightful owner.
6. Hit OK

after doing this, You will now be able to access the Database Diagrams.