Micorosft Sysprep is used.
Sysprep has to be downloaded and extraceted on the vCenter in the location metnioned in the KB
Do vSphere use Microsoft sysprep or does it have a own?
as noted you will have to download Microsoft sysprep if using older Windows versions than 2008 and place those into the correct folder on the vCenter Server. If having the new Linux based vCenter Appliance it has its own internal locations other than specified in the KB.