Dim wb As Microsoft.Office.Interop.Excel.Workbook
Dim xlapp As New Microsoft.Office.Interop.Excel.ApplicationClass
wb = xlapp.Workbooks.Open(Application.StartupPath & "\Rechnung.xls", True, _
  False)
System.Diagnostics.Process.Start(Application.StartupPath & "\Rechnung.xls") _
'"\Rechnung.xls") ', True, False)
Dim ws As Microsoft.Office.Interop.Excel.Worksheet = xlapp.ActiveSheet
Dim s As String = ws.Range("A1").Value
ws.Range("A1").Value = "zzz"

VB@rchiv its the Best