Ok, I solved my problem but thank you anyway.
I am writing you here the solution I used:
Global $items, $msg, $comb, $bot1, $bot2
GUICreate("GUI combo", 180, 110)
$items = "|item1|item2|item3"
$comb = GUICtrlCreateCombo("", 10, 10, 160)
GUICtrlSetData(-1, $items, "item3")
$bot1 = GuiCtrlCreateButton('Selecciona "item1"', 10, 40)
$bot2 = GuiCtrlCreateButton("Selecciona segundo elemento", 10, 70)
GUISetState()
While 1
$msg = GUIGetMsg()
Switch $msg
case $GUI_EVENT_CLOSE
ExitLoop
case $bot1
;GUICtrlSetData($comb, $items, "item1")
GUICtrlSendMsg($comb, $CB_SELECTSTRING, -1, "item1") ; Selecciona por texto
case $bot2
GUICtrlSendMsg($comb, $CB_SETCURSEL, 1, 0) ; Selecciona por posición (0 = primer elemento)
EndSwitch
WEnd