Prova questo:
Private Const MONITOR_ON = -1&
Private Const MONITOR_LOWPOWER = 1&
Private Const MONITOR_OFF = 2&
Private Const SC_MONITORPOWER = &HF170&
Private Const WM_SYSCOMMAND = &H112
Private Declare Function SendMessage Lib "user32" _
Alias "SendMessageA" _
(ByVal hWnd As Long, _
ByVal wMsg As Long, _
ByVal wParam As Long, _
lParam As Any) As Long
'procedura che spegne il monitor
Call SendMessage(Me.hWnd, WM_SYSCOMMAND, SC_MONITORPOWER, ByVal MONITOR_OFF)
Facci sapere...
Ciao