private void button1_Click(object sender, EventArgs e){}

if (textBox1.Text == "Admin" && textBox2.Text == "123123")
             {
                 //Suscesfull Login Message
                 MessageBox.Show("Login Suscesfull,Welcome Admin");
             }

else
             {
                 //Error Messagebox bad login
                 MessageBox.Show("Please check your login informations");
             }

        IniFile ini = new IniFile("C:\\test.ini");
             ini.IniWriteValue("C:\\", "Username", textBox1.Text);
             ini.IniWriteValue("C:\\", "Password", textBox2.Text);

private void Form1_Load(object sender, EventArgs e)
{}

      IniFile ini = new IniFile("C:\\test.ini");
             ini.IniReadValue("C:\\", "Username");//textbox1
             ini.IniReadValue("C:\\", "Password");//textbox2

using System;
using System.Runtime.InteropServices;
using System.Text;

namespace Ini
{
     /// <summary>

     /// Create a New INI file to store or load data

     /// </summary>

     public class IniFile
     {
         public string path;

         [DllImport("kernel32")]
         private static extern long WritePrivateProfileString(string section,
             string key, string val, string filePath);
         [DllImport("kernel32")]
         private static extern int GetPrivateProfileString(string section,
                  string key, string def, StringBuilder retVal,
             int size, string filePath);

         /// <summary>

         /// INIFile Constructor.

         /// </summary>

         /// <PARAM name="INIPath"></PARAM>

         public IniFile(string INIPath)
         {
             path = INIPath;
         }
         /// <summary>

         /// Write Data to the INI File

         /// </summary>

         /// <PARAM name="Section"></PARAM>

         /// Section name

         /// <PARAM name="Key"></PARAM>

         /// Key Name

         /// <PARAM name="Value"></PARAM>

         /// Value Name

         public void IniWriteValue(string Section, string Key, string Value)
         {
             WritePrivateProfileString(Section, Key, Value, this.path);
         }

         /// <summary>

         /// Read Data Value From the Ini File

         /// </summary>

         /// <PARAM name="Section"></PARAM>

         /// <PARAM name="Key"></PARAM>

         /// <PARAM name="Path"></PARAM>

         /// <returns></returns>

         public string IniReadValue(string Section, string Key)
         {
             StringBuilder temp = new StringBuilder(255);
             int i = GetPrivateProfileString(Section, Key, "", temp,
                    255, this.path);
             return temp.ToString();

         }
     }
}