When trying to launch the vSphere Client you receive errors similar to:
Error parsing the server „<servername>“ „clients.xml“ file.
The type initializer for VirtualInfrastructure.Utils.HttpWebRequestProxy‘ threw an exception.
You cannot use vSphere Clients prior to the Update 1 release, to access the vCenter Server or ESX hosts because of a Microsoft update that targets the .NET Framework (980773), released on June 9th 2010.
Perform one of these two options to correct the issue:
Download and install vSphere Client 4.0 Update 1 (build 208111) or Update 2 (build 258672) using method a or b below, depending on your environment.
To download and install the vSphere Client for ESX, ESXi (paid version), and vCenter Server:
Go the downloads site.
Note: The vSphere Client .exe is part of the ESX, ESXi, or vCenter Server download binaries.
Click Download next to your ESX, ESXi, or vCenter Server edition.
Log in with your VMware Account credentials.
Click the .exe link next to vSphere Client and Host Update Utility
Note: You do not need to download the entire vSphere suite, only the vSphere Client.
Follow the on-screen instructions to install the updated vSphere Client.
Read full article here >> http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1022611