Dim objWMIService, objComputer As Object 
objWMIService = GetObject("Winmgmts:{impersonationLevel=impersonate,(Debug,Shutdown)}") 

For Each objComputer In objWMIService.InstancesOf("Win32_OperatingSystem") 
 objComputer.Win32shutdown(12, 0) '12 - выключить компьютер, 2 - перезагрузить
Next 

'полный список значений, которые можно использовать
'0  (0x0)             - Log Off
'4  (0x4)             - Forced Log Off (0 + 4)
'1  (0x1)             - Shutdown
'5  (0x5)             - Forced Shutdown (1 + 4)
'2  (0x2)             - Reboot
'6  (0x6)             - Forced Reboot (2 + 4)
'8  (0x8)             - Power Off
'12  (0xC)          - Forced Power Off (8 + 4)