App with activation interface

This will demo both the file and the app.config approach. I'm reluctant to work on the registry (risking a total crash of the system if one does it wrong).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.IO;
using System.Configuration;

namespace StorePin
{
    class Program
    {
        static void Main(string[] args)
        {
            if (!File.Exists("pinfile.txt"))
            {
                Console.Write("Please enter your pin: ");
                string pin1 = Console.ReadLine();

                using (StreamWriter sw = new StreamWriter("pinfile.txt"))
                {
                    sw.WriteLine(pin1);
                }
            }
            else
            {
                Console.WriteLine("Pin file found");
            }

            Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            KeyValueConfigurationElement kvce = config.AppSettings.Settings["pin"];
            if (kvce == null)
            {
                Console.Write("Please enter your pin: ");
                string pin2 = Console.ReadLine();

                config.AppSettings.Settings.Add("pin", pin2);
                config.Save(ConfigurationSaveMode.Modified);
                ConfigurationManager.RefreshSection("appSettings");
            }
            else
            {
                Console.WriteLine("Pin found in appSettings");
            }

            Console.ReadLine();
        }
    }
}

The second part is based on http://social.msdn.microsoft.com/Forums/vstudio/en-US/77b87843-ae0b-463d-b50e-b6b8e9175e50/write-to-appconfig-at-runtime-very-urgent?forum=csharpgeneral

Note 1: the app.config part might not work as you expect when running from within visual studio because VS copies the original app.Config to the output directory and overwrites the existing (new) configuration there. It will work when running from windows explorer.

Note 2: the program only checks if there is a pinfile or a key in app.Config; for the demo I have written the pin in there. It's not secure, it does not validate if the pin is correct and so on. If it needs to be secure, you need to add some intelligence to the pin and store in encrypted format, decrypt and check if it's a valid pin.