In order to create templates or customize the Windows OS on a cloned virtual machine in vCenter you must add the necessary sysprep files to the proper directory on your vCenter server.
vCenter 4 and up has native support for Windows Server 2008. No sysprep files are necessary.
In Windows Server 2008 the sysprep directory is:
c:\Program Data\VMWare\VMWare Virtual Center\sysprep
In Windows Server 2003 the sysprep directory is:
C:\Documents and Settings\All Users\Application Data\VMWare\VMWare VirtualCenter\sysprep
Choose the appropriate OS for the sysprep files downloaded from Microsoft and extract them to the proper directory using one of the following procedures:
If your sysprep file is an exe file:
(1) Go to "Start, Run"
(2) Enter the path to your sysprep files and run it with a "/x"
ie: c:\sysprep files\WindowsXPxxxxxx.exe /x
This command will run and ask you where to extract the sysprep files. Choose the appropriate sysprep directory.
If your sysprep file is a cab file:
(1) Open your cab file.
(2) Press ctrl-A to select all.
(3) Right click and extract to the appropriate sysprep directory.
*****Here's a link to a KB article from VMWare with more info.*****
VMWare KB Article 1005593