Hello,
You would need to use the System Exec.VI that can be found on the function palette. Its located under Connectivity >> Libraries & Executables palette.
The specific command that you would need to use would be "shutdown".
You can test this command out by opening up the windows command windows (Start >> Run & type in "cmd"). Type "shutdown" to get more information on this command.
Regards,
Chetan Kap
Application Engineering
National Instruments