Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        Select Case e.KeyChar
            Case "a"c To "z"c, "A"c To "Z"c, "0"c To "9", Chr(Keys.Back)
            Case Else
                e.Handled = True
        End Select
    End Sub

Those people who think they know everything are a great annoyance to those of us who do - Isaac Asimov