I installed a Windows 2008 Enterprise Core using Virtual PC 2007 SP1 (which has the Windows 2008 option in the VM creation wizard). Then I tried to install the VM Additions (directly from d:\windows\setup.exe after choosing the VM Additions command of Virtual PC), it always got rolled back.
I found the solution from Jesper Lönnqvist's blog at http://blog.lonnqvist.se/techblog/2008/06/install-virtual.html. Just disable the sound card in the VM settings, boot the VM, and VM additions installation will succeed.
To change the time zone, from the command prompt, type: control timedate.cpl