I've been working at a client the past few weeks and one of my tasks was to begin the process of upgrading their infrastructure to vSphere 5. As we all know, the first step is to upgrade vCenter. So I walked through the upgrade across all three vCenter servers and everything seemed to be ok until we went to deploy a new VM using a customization specification that had been created previously under vCenter 4.1.
During the customization process for the VM we received the infamous "Windows Setup encountered an internal error while loading or searching for an unattended answer file." I opened a ticket with VMware, but I figured out the problem before they could call me back. The problem stems from an amperes and "&" the client was using in the Organization field on the first screen of the Guest Customization wizard. A simple change of "&" to "and" fixed the issue and everyone is happy.
VMware did call back and I asked them to reproduce and verify that we were not the only customer to run into this. They said that we were only the second customer to open a ticket and report the issue, but that they would create a KB article so that if someone else ran into the issue the information would be out there.
I hope this helps anyone who has been scratching their head over this small, but potentially significant issue.