You cannot use vSphere 4 Clients prior to the Update 1 release to access the vCenter Server or ESX hosts because of a Microsoft update that targets the Microsoft .NET Framework (980773) released on June 9th 2010
You cannot use vSphere 4 Clients prior to the Update 1 release to access the vCenter Server or ESX hosts because of a Microsoft update that targets the Microsoft .NET Framework (980773) released on June 9th 2010