Как узнать что компьютер заблокирован

Можно время от времени выполнять этот код и проверять статус сессии:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;
namespace EnumerateRDUsers
{
  class Program
  {
    [DllImport("wtsapi32.dll")]
    static extern IntPtr WTSOpenServer([MarshalAs(UnmanagedType.LPStr)] String pServerName);
    [DllImport("wtsapi32.dll")]
    static extern void WTSCloseServer(IntPtr hServer);
    [DllImport("wtsapi32.dll")]
    static extern Int32 WTSEnumerateSessions(
        IntPtr hServer,
        [MarshalAs(UnmanagedType.U4)] Int32 Reserved,
        [MarshalAs(UnmanagedType.U4)] Int32 Version,
        ref IntPtr ppSessionInfo,
        [MarshalAs(UnmanagedType.U4)] ref Int32 pCount);
    [DllImport("wtsapi32.dll")]
    static extern void WTSFreeMemory(IntPtr pMemory);
    [DllImport("Wtsapi32.dll")]
    static extern bool WTSQuerySessionInformation(
        System.IntPtr hServer, int sessionId, WTS_INFO_CLASS wtsInfoClass, out System.IntPtr ppBuffer, out uint pBytesReturned);
    [StructLayout(LayoutKind.Sequential)]
    private struct WTS_SESSION_INFO
    {
      public Int32 SessionID;
      [MarshalAs(UnmanagedType.LPStr)]
      public String pWinStationName;
      public WTS_CONNECTSTATE_CLASS State;
    }
    public enum WTS_INFO_CLASS
    {
      WTSInitialProgram,
      WTSApplicationName,
      WTSWorkingDirectory,
      WTSOEMId,
      WTSSessionId,
      WTSUserName,
      WTSWinStationName,
      WTSDomainName,
      WTSConnectState,
      WTSClientBuildNumber,
      WTSClientName,
      WTSClientDirectory,
      WTSClientProductId,
      WTSClientHardwareId,
      WTSClientAddress,
      WTSClientDisplay,
      WTSClientProtocolType
    }
    public enum WTS_CONNECTSTATE_CLASS
    {
      WTSActive,
      WTSConnected,
      WTSConnectQuery,
      WTSShadow,
      WTSDisconnected,
      WTSIdle,
      WTSListen,
      WTSReset,
      WTSDown,
      WTSInit
    }
    static void Main(string[] args)
    {
      ListUsers("<INSERT SERVERNAME HERE>");
    }
    public static IntPtr OpenServer(String Name)
    {
      IntPtr server = WTSOpenServer(Name);
      return server;
    }
    public static void CloseServer(IntPtr ServerHandle)
    {
      WTSCloseServer(ServerHandle);
    }
    public static void ListUsers(String ServerName)
    {
      IntPtr serverHandle = IntPtr.Zero;
      List<String> resultList = new List<string>();
      serverHandle = OpenServer(ServerName);
      try
      {
        IntPtr SessionInfoPtr = IntPtr.Zero;
        IntPtr userPtr = IntPtr.Zero;
        IntPtr domainPtr = IntPtr.Zero;
        Int32 sessionCount = 0;
        Int32 retVal = WTSEnumerateSessions(serverHandle, 0, 1, ref SessionInfoPtr, ref sessionCount);
        Int32 dataSize = Marshal.SizeOf(typeof(WTS_SESSION_INFO));
        IntPtr currentSession = SessionInfoPtr;
        uint bytes = 0;
        if (retVal != 0)
        {
          for (int i = 0; i < sessionCount; i++)
          {
            WTS_SESSION_INFO si = (WTS_SESSION_INFO)Marshal.PtrToStructure((System.IntPtr)currentSession, typeof(WTS_SESSION_INFO));
            currentSession += dataSize;
            WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSUserName, out userPtr, out bytes);
            WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSDomainName, out domainPtr, out bytes);
            Console.WriteLine("Domain and User: " + Marshal.PtrToStringAnsi(domainPtr) + "\\" + Marshal.PtrToStringAnsi(userPtr));
            WTSFreeMemory(userPtr); 
            WTSFreeMemory(domainPtr);
          }
          WTSFreeMemory(SessionInfoPtr);
        }
      }
      finally
      {
        CloseServer(serverHandle);
      }
    }
  }
}