Esse código faz com que a TextBox só aceite numeros e o backspace.

    Private Sub TxtTextBox_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TxtTextBox.KeyPress

        Dim KeyAscii As Byte = Convert.ToByte(e.KeyChar)

        If KeyAscii <> 8 Then

            If KeyAscii > 47 And KeyAscii < 58 Then

                e.Handled = False

            Else

                e.Handled = True

            End If

        End If

    End Sub