This happens in Vista when you try to create a database in microsoft sql 2008. The reason is that by default the administrator account does not have enough rights.

The solution is to grant rights to the administrator

Step 1: Go to Start > All Programs > Microsoft SQL Server 2005 > Configuration Tools > SQL Server Surface Area Configuration.

Step 2: Click on ‘Add New Administrator’.

Step 3: In the ‘SQL Server User Provisioning on Vista’ dialog box, look out for the ‘Member of the SqlServer SysAdmin role ‘ in the ‘Available Privileges’ box. Transfer it to the ‘Privileges that will be granted to’ box. Click Ok.

and at last

Control Panel->User Accounts->Turn User Account Control on or off. turn it off and try again