If you are trying to run older version of VMware vSphere Client on Windows 7, or fully updated Windows XP you may get following errors:

VMware vSphere Client error – Error parsing the server “[server]” “clients.xml” file

The type initializer for VirtualInfrastructure.Utils.HttpWebRequestProxy’ threw an exception.

These errors are caused by an updated Microsoft .NET version.

You have couple of options here.
Probably easiest fix is to download latest version of vSphere Client from VMware. If for some reason you can’t do this follow instructions below

1. Download system.dll file. This file is taken from older version of Microsoft .NET installation.
2. Copy this file to C:\Program Files\VMware\Infrastructure\Virtual Infrastructure Client\Launcher\lib
   On 64 bit OS path would be: C:\Program Files (x86)\VMware\Infrastructure\Virtual Infrastructure Client\Launcher\lib
   If lib folder doesn’t exist then create it.
3. Open file C:\Program Files\VMware\Infrastructure\Virtual Infrastructure Client\Launcher\VpxClient.exe.config in a text editor and just before last line </configuration> paste following code:
   <runtime>
   <developmentMode developerInstallation=”true”/>
   </runtime>
4. Control Panel > System > Advanced > Environment Variables
   In System Variables click New and add following system variable:
   Name: DEVPATH
   Value: C:\Program Files\VMware\Infrastructure\Virtual Infrastructure Client\Launcher\lib
   Remember that on 64 bit system instead of Program Files you have to use Program Files (x86)
5. Launch VMware vSphere Client again. This time it should run without any errors.