Для перезагрузки

Dim strComputer As String 
strComputer = "."
Set objWMIService = GetObject("winmgmts:" & "{impersonationLevel=impersonate, (Shutdown)}!\\" & strComputer & "\root\cimv2")
Set colOperatingSystems = objWMIService.ExecQuery( "Select * from Win32_OperatingSystem")
For Each ObjOperatingSystem In colOperatingSystems
ObjOperatingSystem.Reboot
Next

Для выключения

Dim strComputer As String
strComputer = "."
Set objWMIService = GetObject("winmgmts:" & "{impersonationLevel=impersonate,(Shutdown)}!\\" & strComputer & "\root\cimv2")
Set colOperatingSystems = objWMIService.ExecQuery( "Select * from Win32_OperatingSystem")
For Each ObjOperatingSystem In colOperatingSystems
ObjOperatingSystem.ShutDown
Next