Ran into a problem today with the Customization Specifications when trying to deploy a Windows Server 2008 VM Template using ESX Server 3.5 and VirtualCenter 2.5 Update 3. I got an error message stating:

Warning: Customization of the guest operating system ‘winLonghornGuest’ is not supported in this configuration.

Checking on the VMware communities site, I came across this discussion in which an administrator was able to get by this by changing the Guest OS type to Windows Vista as shown below:

Once the VM is created, edit the settings and change the Guest OS back to Windows Server 2008.

If there are any negatives to doing this, I’d love to hear them.