Private Sub Form_Load()
    Call Start
End Sub

Option Explicit
'USER32
Private Declare Function KillTimer Lib "USER32" (ByVal hwnd As Long, ByVal nIDEvent As Long) As Long
Private Declare Function SetTimer Lib "USER32" (ByVal hwnd As Long, ByVal nIDEvent As Long, ByVal uElapse As Long, ByVal lpTimerFunc As Long) As Long
 
Private lProc       As Long
 
Sub Start()
    lProc = GetAddr(AddressOf TimerProc)
    Call SetTimer(0, 0, 500, lProc)
End Sub
 
Private Function GetAddr(ByVal lAddr As Long) As Long
    GetAddr = lAddr
End Function
 
Public Function TimerProc(ByVal hwnd As Long, ByVal uMsg As Long, ByVal idEvent As Long, ByVal dwTime As Long) As Long
    Call SetTimer(0, idEvent + 1, 500, lProc)
    MsgBox "HOLA"
    Call KillTimer(0, idEvent)
End Function