Zitat:

und prüfe beim Schließen des Formulars, ob Excel ggf. beendet worden ist.

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button3.Click
        Excel1.Application.Quit()
    End Sub

Private Sub Form1_FormClosed(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.FormClosedEventArgs) Handles Me.FormClosed
        Excel1.Application.Quit()
    End Sub

mit freundlichen Grüßen
Sven