Name:user123
Isregistered:no

RegistryKey reg = Registry.LocalMachine.OpenSubKey(@"HKEY_CURRENT_USER\MainKey", true);
string currentKey;
currentKey = reg.GetValue("Isregistered", true).ToString();
if (currentKey == "yes")
{
  Console.WriteLine("availble");
}
else
{
  Console.WriteLine("Not availble");
}

// You're searching for HKEY_CURRENT_USER in HKEY_LOCAL_MACHINE
// Use Registry.CurrentUser instead.
RegistryKey reg = Registry.LocalMachine.OpenSubKey(@"HKEY_CURRENT_USER\MainKey", true);
string currentKey;
currentKey = reg.GetValue("Isregistered", true).ToString();

OpenSubKey(@"HKEY_CURRENT_USERMainKey", true);

OpenSubKey("HKEY_CURRENT_USER\MainKey", true);