Const SM_CXSCREEN = 0 'X Size of screen
Const SM_CYSCREEN = 1 'Y Size of Screen
Private Declare Function GetSystemMetrics Lib "user32" (ByVal nIndex As Long) As Long
despues sigues la formula:
x=ancho/2
y=alto/2
y aplicas un Setcursorpos!¡.
P.D.: si no sabes que onda revisa la MSDN.
Dulces Lunas!¡.