What needs to be installed prior to the vCenter Server depends on whether you want to use SQL Express and whether you use a non-English Windows version. Please take a look at http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-installation-setup-guide.pdf?lang=en (vCenter Server Software Requirements - page 37).
André