Use this
private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
{
fsttxt.Text = dataGridView1.SelectedCells[0].Value.ToString();
lsttxt.Text = dataGridView1.SelectedCells[1].Value.ToString();
usertxt.Text = dataGridView1.SelectedCells[2].Value.ToString();
passtxt.Text = dataGridView1.SelectedCells[3].Value.ToString();
}