GUICtrlCreateLabel creates a label 
GUICtrlSetData updates the text of a previously created label (or other controls) 
you are creating thousands of new controls the longer your script runs. 
 
also you ping spam the website with no delay between the pings. 
added a timer which pings only every second and fixed updating the label: 
 
 #include <GUIConstantsEx.au3>
#include <Array.au3>
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
#include <SendMessage.au3>
Box()
Func Box()
    Local $msg
    GUICreate("iSerius", 260, 85, 1093, 625)
    GUICtrlCreateLabel("iSerius : ", 5, 5, 100, 36)
    $btnCon = GUICtrlCreateButton("Connect", 10, 50, 50, 25, 0)
    $btnDC = GUICtrlCreateButton("Disconnect", 62, 50, 75, 25, 0)
    $btnMCE = GUICtrlCreateButton("MCE", 190, 50, 40, 25, 0)
    $lStatus = GUICtrlCreateLabel("UP", 40, 5, 100, 36)
    GUISetState()
    $ti = TimerInit()
    While 1
        If TimerDiff($ti) > 1000 then
            $ti = TimerInit()
            If Ping("google.com", 1000) Then
                GUICtrlSetData($lStatus,"UP")
            Else
                GUICtrlSetData($lStatus,"DOWN")
            EndIf
        EndIf
        $msg = GUIGetMsg()
        Select
            Case $msg = $GUI_EVENT_CLOSE
                Exit
            Case $msg = $GuI_EVENT_MINIMIZE
            Case $msg = $GUI_EVENT_MAXIMIZE
            Case $msg = $btnCon
                Run("rasphone -d iSerius")
            Case $msg = $btnDC
                Run("rasdial /DISCONNECT")
            Case $msg = $btnMCE
                Run("explorer.exe 10.1.1.1")
        EndSelect
    WEnd
EndFunc   ;==>Box