Public Class Form1
 
    Public Sub New()
        InitializeComponent()
 
    End Sub
 
    Dim Excel1 As New Microsoft.Office.Interop.Excel.Application()
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
 
        If My.Computer.FileSystem.FileExists(TextBox1.Text) Then
            Excel1.Workbooks.Open(TextBox1.Text)
            Excel1.Visible = True
 
        Else
            MsgBox("Datei existiert nicht", MsgBoxStyle.Critical, "Fehler")
            CloseThis()
            Exit Sub
        End If
    End Sub
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button3.Click
        CloseThis()
 
    End Sub
 
    Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
        CloseThis()
    End Sub
 
    Private Sub CloseThis()
 
        Excel1 = Nothing
 
        GC.Collect()
        GC.WaitForPendingFinalizers()
    End Sub
 
End Class

mit freundlichen Grüßen
Sven