When you using vSphere 4 and and using MDT 2012 update1 with Windows Assessment and Deployment Kit you run in de following error. Same thing wil also with Workstation 8.
Solution 1: Do not install Windows Assessment and Deployment Kit on your MDT Server. You can still using MDT 2012 Update 1 with waik 3.5 if not deploying Windows 8 or Windows 2012 right now.
Solution 2: Upgrading you vSphere server to the latest version (5.1) or Upgrade Workstation to latest version (Version 9).