You can use a scheduled task to accomplish this.
Download the sysinternals psshutdown.exe command.
http://technet.microsoft.com/en-us/sysinternals/bb897541.aspx
Place the file within c:\windows
Schedule a task as the local administrator
c:\windows\psshutdown.exe -o -f
Under the schedule tab > Schedule Task: select "When idle"
Input the minutes in the box: "When the compute has been idle for:
Under the Settings tab:
Levae the 72 hours at the top as is.
Select "Only start the task if the computer has been idle for at least:
<input the number of minutes you want>
"if the computer has not been idle that long, retry for up to:
<you can leave this at zero>
Leave all other boxes beyond this point empty.
Play with the settings to adjust them as you deem necassary