Compile and run it as scheduled task - it fails here! It does not fail when run logged on.

regtest.cs

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.Win32;
using System.IO;
using System.Security.Principal;

namespace regtest
{
    class Program
    {
        static void Main(string[] args)
        {
            StreamWriter sw = new StreamWriter("regtest.log");
            try
            {
                RegistryKey HKLM = Registry.LocalMachine;
                try
                {
                    WindowsPrincipal principal = new WindowsPrincipal(WindowsIdentity.GetCurrent());
                    string isAdmin = principal.IsInRole(WindowsBuiltInRole.Administrator) ? "Yes" : "No";
                    sw.WriteLine("Is the user admin: " + isAdmin);

                    // Apply Your path here!!
                    RegistryKey General = HKLM.OpenSubKey("your\\path\\here", false);
                    try
                    {
                        // Apply Your setting!!
                        sw.WriteLine("OK: " + General.GetValue("@YOUR KEY@")");
                    }
                    catch (Exception c)
                    {
                        sw.WriteLine("Could not read key: " + c.ToString());
                    }
                }
                catch (Exception b)
                {
                    sw.WriteLine("Could not open path: " + b.ToString());
                }
            }
            catch (Exception a)
            {
                sw.WriteLine("Could not open HKLM: " + a.ToString());
            }

            sw.Close();
        }
    }
}