hi,
try like this:
public void FillComboBox()
{
String ConnStr = "Data Source=ServerName;Initial Catalog=DBNamae;User ID=UserName;Password='password';";
String SQL = "SELECT * FROM Countries";
SqlDataAdapter ad = new SqlDataAdapter(SQL, ConnStr);
DataSet ds = new DataSet();
ad.Fill(ds);
comboBox1.DataSource = ds.Tables[0];
comboBox1.DisplayMember = "CountryName";
}