A few people have notice that since upgrading to Virtualcenter 2.5 deploying a Windows 2003 VM from template no longer gives them the ability to customize. The option is disabled.
The fix for this is simple. You need to grab the new deployment tools from Microsoft’s site here also referred to as the sysprep files. You will be downloading WindowsServer2003-KB892778-SP1-DeployTools-x86-ENU.cab
This file needs to be extracted on the Virtualcenter server and placed in the following folder-
All Users\Application Data\Vmware\VirtualCenter\sysprep\svr2003
Once these files exist in the correct folder, you will be able to deploy your W2K3 template.
For more details read the Virtual Center 2 Template Usages and Best Practices http://www.vmware.com/pdf/vc_2_templates_usage_best_practices_wp.pdf