Function Search(ByVal Suchwort As String, Optional ByVal StartSearch As _
  Integer=0) As Integer
   Dim BeginSearchIndex As Integer = 0 ' für Weitersuchen
   Dim stInd As Integer = RichTextBox1.Text.IndexOf(Suchwort,StartSearch)
   RichTextBox1.SelectionStart = stInd
   RichTextBox1.SelectionLength = Suchwort.Length
End Function

Private lastFoundIndex As Integer = 0
Private Sub btnSearch_Click(sender As Object,e As System.EventArgs)Handles _
  btnSearch.Click
   lastFoundIndex = Search(txtSuchwort.Text,lastFoundIndex)
End Sub