Database diagram support objects cannot be installed because this database does not have a valid owner.

I have restore a database from MSSQL 2000 to MSSQL 2005. When i try to open the database diagram, error message below pop up:

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.

ALTER AUTHORIZATION ON DATABASE: Database_name TO valid_login

I use SSMS to solve my problem.
In SQL Server Management Studio do the following:

1. Right Click on your database, choose properties
2. Go to the Options
3. In the Dropdown at right labeled "Compatibility Level" choose "SQL Server 2005(90)"
4. Go to Files
5. Type "sa" in the owner textbox
6. Click OK

This solve my problem.

Alternatively, we can solve it using TSQL:
- My database name: avlDB
- MY Login: sa

EXEC sp_dbcmptlevel 'avlDB', '90';
GO
ALTER AUTHORIZATION ON DATABASE::avlDB TO "sa"
GO
USE [avlDB]
GO
EXECUTE AS USER = N'dbo' REVERT
GO