Essaie avec cette fonction (je ne l'ai pas testée, mais normalement ça marche)
Const SPI_SETSCREENSAVEACTIVE = 17
Declare Function SystemParametersInfo Lib 'user32.dll' Alias _
'SystemParametersInfoA' (ByVal uAction As Long, ByVal uiParam As Long, _
pvParam As Any, ByVal fWinIni As Long) As Long
Sub ActiveScreenSaver(OnOff As Boolean) 'Ti
Dim Ret&
Ret = SystemParametersInfo(SPI_SETSCREENSAVEACTIVE, OnOff, 0, 0)
End Sub