Permissions Needed To Attach AdventureWorks Database
You have not given your admin user the correct permissions yet. You must explicitally tell SQL Server that this user is trusted. Running the program as an admin gives the program itself the highest level of permissions, but does nothing for the authentication process when executng a query.  

To add your administrator account as a SQL Administrator:


Open 'SQL Server Surface Area Configuration'  (Under Start -> MS SQL Server -> Configuration Tools)
Click "Add New Administrator"
Move the user from the left to the right, then save changes.


In 2008 and later, follow the instructions given here.