http://blogs.msdn.com/saurabhkv/archive/2009/06/24/iis-service-unavailable-error-on-a-64-bit-operating-system.aspxPROBLEM: If you find this error in the event log when starting an iis service on a 64 bit operating system (personally this happened to us with SharePoint). (This is also followed by a Service Unavailable error when trying to use the application): Eventen-USCommunityServer 2.1 SP1 (Build: 61025.2)