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).