Simple way
Private Sub Application_Reminder(ByVal Item As Object)
If TypeOf Item Is AppointmentItem Then
MsgBox "Message text", vbSystemModal, "Message title"
End If
End Sub
(source)
Sophisticated way
Private Declare PtrSafe Function FindWindowA Lib "user32" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare PtrSafe Function SetWindowPos Lib "user32" ( _
ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
ByVal X As Long, ByVal Y As Long, ByVal cx As Long, _
ByVal cy As Long, ByVal wFlags As Long) As Long
Private Const SWP_NOSIZE = &H1
Private Const SWP_NOMOVE = &H2
Private Const FLAGS As Long = SWP_NOMOVE Or SWP_NOSIZE
Private Const HWND_TOPMOST = -1
Private Sub Application_Reminder(ByVal Item As Object)
Dim ReminderWindowHWnd As Variant
On Error Resume Next
ReminderWindowHWnd = FindWindowA(vbNullString, "1 Reminder")
SetWindowPos ReminderWindowHWnd, HWND_TOPMOST, 0, 0, 0, 0, FLAGS
End Sub
(source)