On Microsoft Windows Vista, you may find yourself unable to access your SQL Server instance, even if the user is in the local admin group.  This is due to new security concerns in Windows Vista.  In SQL Server 2005 Service Pack 2, Microsoft released a tool called the User Provisioning Tool for Vista.

This tool allows you to give administrative permissions to users.  The tool should launch when you upgrade to Service Pack 2, however if it encounters an error or for some reason you find yourself unable to access your SQL Server, give them rights in the User Provisioning Tool by opening the SQL Server 2005 Surface Area Configuration tool (usually found under Microsoft SQL Server 2005 >> Configuration Tools on the start menu), and clicking Add New Administrator. 

After that, simply add each user/server to the granted rights box on the right to give that user admin rights.