Can’t create add a SQL Server user: The login already has an account under a different user name

Environment:

• SQL Server 2005 Express
• Windows 7

When I installed SQL Server, I followed the instructions at http://msdn.microsoft.com/en-us/library/aa905868.aspx to set my computer's admin account as the SQL Server admin.

However, when I try to access a database on my computer through Visual Studio 2008, I get the following error message:

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Microsoft Visual Studio
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The database 'Parkinsons' does not exist or you do not have permission to see it.



Would you like to attempt to create it?
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Yes   No   
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Then, if I go to SQL Server and add a user to that database, I get the following error message:

TITLE: Microsoft SQL Server Management Studio Express
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Create failed for User 'zian'.  (Microsoft.SqlServer.Express.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=9.00.2047.00&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Create+User&LinkId=20476

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ADDITIONAL INFORMATION:

An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.Express.ConnectionInfo)

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The login already has an account under a different user name. (Microsoft SQL Server, Error: 15063)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=09.00.4053&EvtSrc=MSSQLServer&EvtID=15063&LinkId=20476

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BUTTONS:

OK
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1. Why doesn't VS piggy back on the dbo account?
2. If the dbo account is unusable, then why won't SQL Server let me make an account so that I can access my own data?