I've had trouble with computers that are joined to my domain and have IP v6 installed if they are running Vista SP1 or Server 2008. The problem goes away if you disable IP v6. Is there a group policy to disable IP v6?
Curt ~ Upgrade to Next Generation Antispam/Antivirus with Ninja! ~ ~ <http://www.sunbelt-software.com/SunbeltMessagingNinja.cfm> ~