C#

<pre>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{


    public class RegistryTester
    {
       

        const int INFINITE = -1;
        const int WAIT_OBJECT_0 = 0;
        const long HKEY_LOCAL_MACHINE = 0x80000002L;
        const long REG_NOTIFY_CHANGE_LAST_SET = 0x00000004L;

        [DllImport("advapi32.dll", EntryPoint = "RegNotifyChangeKeyValue")]
        static extern long RegNotifyChangeKeyValue(IntPtr key,
            bool watchSubTree, int notifyFilter, IntPtr regEvent, bool
            async);

        [DllImport("advapi32.dll", EntryPoint = "RegOpenKey")]
        static extern IntPtr RegOpenKey(IntPtr key, String subKey,
            out IntPtr resultSubKey);

        [DllImport("kernel32.dll", EntryPoint = "CreateEvent")]
        static extern IntPtr CreateEvent(IntPtr eventAttributes,
            bool manualReset, bool initialState, String name);

        [DllImport("kernel32.dll", EntryPoint = "WaitForMultipleObjects")]
        static extern unsafe int WaitForMultipleObjects(int numHandles,
            IntPtr* handleArrays, bool waitAll, int timeOut);

        [DllImport("kernel32.dll", EntryPoint = "CloseHandle")]
        static extern IntPtr CloseHandle(IntPtr handle);

        public unsafe static void Main(String[] args)
        {
            args = new string[] { "1", @"SYSTEM\CurrentControlSet\Control\Test" };
            

            int ret = 0;

            if (args.Length < 2)
            {
                Console.WriteLine("Usage: RegistryTester NUMKEYS KEY1 KEY2...");
                return;
            }

            int numKeys = 0;

            try
            {
                numKeys = int.Parse(args[0]);
            }
            catch (Exception)
            {
                Console.WriteLine("Invalid argument for NUMKEYS");
                return;
            }

            if (numKeys != args.Length - 1)
            {
                Console.WriteLine("Did not provide correct number of key arguments.");
                return;
            }

            String[] keys = new String[numKeys];

            for (int i = 0; i < numKeys; i++)
            {
                keys[i] = args[i + 1]; // Start at the 3rd command line arg
            }

            IntPtr[] eventHandles = new IntPtr[numKeys];

            for (int i = 0; i < numKeys; i++)
            {
                eventHandles[i] = CreateEvent((IntPtr)null, false, false,
                    null);

                IntPtr myKey;

                unchecked
                {
                    RegOpenKey(new IntPtr((int)HKEY_LOCAL_MACHINE),
                        keys[i], out myKey);
                }

                RegNotifyChangeKeyValue(myKey, true,
                    (int)REG_NOTIFY_CHANGE_LAST_SET, eventHandles[i],
                     true);
            }

            Console.WriteLine("Waiting on " + numKeys + " keys.");

            fixed (IntPtr* handlePtr = eventHandles)
            {
                ret = WaitForMultipleObjects(numKeys, handlePtr, false,
                    INFINITE);
            }

            Console.WriteLine(keys[ret - WAIT_OBJECT_0] + " was changed.");
            Console.ReadLine();
        }
    }
}

private void button2_Click(object sender, EventArgs e)
{
   // Create ttimer
    System.Timers.Timer Timer;
    Timer = new System.Timers.Timer();
    Timer.Interval = 20; // Timer Intervall = 20 ms
    Timer.Enabled = true;

    Timer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
}

private void OnTimedEvent(object source, ElapsedEventArgs e)
{
    // Invoke, since it`s anoter thread (?)
    Invoke(new MethodInvoker(delegate
    {
        // add 20 to the value in the textbox
        label1.Text = (Convert.ToInt32(label3.Text) + 20).ToString();
    }));
}