If you've been playing around with Vista, you've already hit this problem, but I see it a lot so I thought I'd add it here.
Vista is a more secure OS, and acts a little like a UNIX box. When you're an administrator in Vista, your group membership doesn't pass through to applications automatically. You're either asked to continue as an admin, or you're silently blocked. This second behavior is how the Management Studio application works in Vista.
What that means is that even though you're an administrator on your Vista system, Management Studio doesn't know that. So when you connect to your SQL Server,it doesn't find your name, or your group membership, and you're not allowed in.
There are three fixes. The first is that only SQL Server SP2 is "supported" on Vista. It has a step in upgrade to put you in the right groups.
The second fix, which isn’t a great one, mind you, is to right-click Management Studio and run it as an administrator. Then everything works like it used to.
An even better solution is to put your Windows account specifically in the sysadmins group in SQL Server. This is effectively what the SP2 tool does.
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