Option Explicit
 
Private Declare Function SetCapture Lib "user32" _
  (ByVal hwnd As Long) As Long
 
Private Declare Function ReleaseCapture Lib "user32" _
  () As Long
 
Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y _
  As Single)
  If X >= 0 And Y >= 0 And X <= Me.Width And Y <= Me.Height Then
    SetCapture Me.hwnd
  Else
    ReleaseCapture
    MsgBox "Maus außerhalb Form!"
  End If
End Sub