Ok wogdog,
You can use Software Delivery to meet the goal, so I will provide some steps for apply this:
(I'm consideer that you're using Sofware Delivery 6.x)
1) Create software delivery task to execute a command line:
shutdown -s -f -c "Altiris automatic shutdown"
These parameters mains:
-s Shutdown the computer
-f Forces running applications to close without warning
-c Shutdown comment
The result will be the following popup:
Additional configurations that could be useful:
2) Configure the task to use the System credencial
3) Configure the schedule at regular basis according your needs, at "Run on a schedule"
4) Mark the option "Warn the user before running this task"
Regards,