If you have a problem customising a Windows 2003 Server template when deploying after upgrading to Virtual Center 2.5 then follow these steps:

To fix this you need to download the new sysprep files

Extract them on your VirtualCenter server and placed in the following folder: %SYSTEMDRIVE%\All Users\Application Data\Vmware\VirtualCenter\sysprep\svr2003

Once extracted the template should now work again.

For more details read the Virtual Center 2 Template usage and Best Practices found here: Usage and Best Practice Guide