textBox.KeyPress += new KeyPressEventHandler(textBox_KeyPress);

private void textBox_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar < 65 || e.KeyChar > 122)
    {
        e.Handled = true;
    }
}

if (!Regex.IsMatch(textbox.Text, @"[a-zA-Z]"))
{
  yourErrorProvider.setError(textbox, "Only A-Z accepted.");
}

if ((Convert.ToInt32(e.KeyChar) >= 65) && (Convert.ToInt32(e.KeyChar) <= 122))

            {
                errorProvider1.SetError(textBox1, "OK");
                errorProvider2.SetError(textBox1, "");
            }
            else
            {
                errorProvider2.SetError(textBox1, "Can not enter the numbers...");
                errorProvider1.SetError(textBox1, "");
                textBox1.Text = "";
            }