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 
        Label1.Caption = "Aktives Fenster: " & Left$(sText, _
        GetWindowText(lHwnd, ByVal sText, 255))
 
    Else 
        Label1.Caption = "Aktives Fenster: Form1"
    End If 
 
End Sub