What OS? Is it a domain user added to the local admins group? I can’t reproduce it on Win7 with a local user which is in the local admins group. But i have a Windows Server 2008 (same as Vista) which has a domain user added to local admins via GPO and it always asks for confirmation of admin tasks, even when this user is already an admin. Maybe this is same issue here. Maybe also something in the logs.