Private Declare Function GetForegroundWindow Lib "user32" () As Long
Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, ByVal cch As Long) As Long

Private Sub Form_Load()
Timer1.Interval = 100
End Sub

Private Sub Timer1_Timer()
Static lHwnd As Long
Dim lCurHwnd As Long
Dim sText As String * 255
lCurHwnd = GetForegroundWindow
If lCurHwnd = lHwnd Then Exit Sub
lHwnd = lCurHwnd
If lHwnd <> hwnd Then
Caption = "Ventana Activa: " & Left$(sText, GetWindowText(lHwnd, ByVal sText, 255))
Else
Caption = "Ventana Activa: Form1"
End If
End Sub