+ $WS_MAXIMIZEBOX?
Edit:
modifiziertes Beispiel aus der Hilfe:
#include <GUIConstants.au3>
GUICreate("My GUI", Default, Default, Default, Default, $WS_MAXIMIZEBOX + $WS_MINIMIZEBOX) ; will create a dialog box that when displayed is centered
GUISetState (@SW_SHOW) ; will display an empty dialog box
; Run the GUI until the dialog is closed
While 1
$msg = GUIGetMsg()
If $msg = $GUI_EVENT_CLOSE Then ExitLoop
Wend
peethebee