When installing vCenter 4.1 on Windows 2008 R2 64-bit one of my colleagues ran into the following error message:

This product can only be installed on the following 64-bit operating systems:
Windows XP SP2 or above
Windows 2003
Windows 2008

Although this message is actually correct it was not what was causing this problem as he followed the documentation and installed Windows 2008 64-bit. In this case Active Directory had been installed and that was the reason it was failing. As vCenter installs ADAM it can’t run on top of a server which hosts AD.