Can't run a Run Script task on Windows 2008 with Local System Account.
Steps to duplicate:
This is working as designed. This is a Microsoft security feature introduced with Windows 2008 & Vista. When you do not specify a user, the task will run as session 0, and session 0 does not have direct access to the desktop.
You can run the script using any of the following work around
DS 9.6 SP3