private void btnCount_Click(object sender, EventArgs e)
{
    label1.Visible=true;
    progressBar.Visible = true;
    progressBar.Minimum = 0;
    progressBar.Maximum = ds.Tables["表"].Rows.Count;
    progressBar.BackColor = Color.Green;
    for (int i = 0; i < ds.Tables["表"].Rows.Count; i++)
    {
        progressBar.Value++;
        Application.DoEvents();
        this.label1.Text = Convert.ToString(progressBar.Value);
    }
}

或者

private void btnCount_Click(object sender, EventArgs e)
{
    label1.Visible=true;
    progressBar.Visible = true;
    progressBar.Minimum = 0;
    progressBar.Maximum = ds.Tables["表"].Rows.Count;
    progressBar.BackColor = Color.Green;
    for (int i = 0; i  < ds.Tables["表"].Rows.Count; i++)
    {
        progressBar.Value++;
        Application.DoEvents();
        this.label1.Text = Convert.ToString(progressBar.Value);this.label1.Refresh();
    }
}