Jay,
.Net doesn't support Global hooks (http://support.microsoft.com/kb/318804/),
which is the only way I know of to do exactly what you are asking.  However,
you should be able to use a workaround if your application doesn't need to
interject/handle the actual mouse click event.  The attached cllass will
allow you to register an event handler and wait for the handle.  It works off
a timer set to 500ms which may/may not be acceptable for you.  Either way,
you can modify it to fit your needs.

Jared

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace NewsgroupWindowsApp
{
   public sealed class TaskBarMonitor : IDisposable
   {

       #region Ctors/Finalizer

       static TaskBarMonitor()
       {
           _instance = new TaskBarMonitor();
       }

       private TaskBarMonitor()
       {
           _foregroundWindow = Win32.GetForegroundWindow();
           _timer = new System.Timers.Timer(500);
           _timer.Elapsed += new
System.Timers.ElapsedEventHandler(_timer_Elapsed);
           _timer.Start();
       }

       ~TaskBarMonitor()
       {
           this.Dispose();
       }

       #endregion

       #region Fields

       private static TaskBarMonitor _instance;
       private static IntPtr _foregroundWindow;
       private static System.Timers.Timer _timer;

       #endregion

       #region Properties

       public static TaskBarMonitor Instance
       {
           get { return _instance; }
       }

       #endregion

       #region Delegates/Events
       
       public delegate void ForegroundWindowChangedHandler(object sender,
ForgroundWindowChangedEventArgs e);
       public event ForegroundWindowChangedHandler ForgroundWindowChanged;

       #endregion

       #region Methods/Handlers

       private void OnForegroundWindowChanged()
       {
           if (ForgroundWindowChanged != null)
           {
               ForgroundWindowChangedEventArgs e = new
ForgroundWindowChangedEventArgs(_foregroundWindow);
               ForgroundWindowChanged(this, e);
           }
       }
       
       void _timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
       {
           IntPtr currentWindow = Win32.GetForegroundWindow();
           if (currentWindow.Equals(_foregroundWindow) == false)
           {
               _foregroundWindow = currentWindow;
               this.OnForegroundWindowChanged();
           }
       }

       #endregion

       #region IDisposable Members

       public void Dispose()
       {
           _timer.Dispose();
       }

       #endregion

       public class ForgroundWindowChangedEventArgs
       {
           #region Ctors

           public ForgroundWindowChangedEventArgs(IntPtr windowHandle)
           {
               _title = this.GetTitle(windowHandle);
           }

           #endregion

           #region Fields

           private string _title;

           #endregion

           #region Properties

           public string Title {
               get
               {
                   if (string.IsNullOrEmpty(_title))
                   {
                       return "Unknown";
                   }
                   return _title;
               }
           }

           public IntPtr Handle
           {
               get { return _foregroundWindow; }
           }

           #endregion

           #region Methods

           private string GetTitle(IntPtr windowHandle)
           {
               StringBuilder sb = new StringBuilder(255);
               Win32.GetWindowText(windowHandle, sb, sb.Capacity);
               return sb.ToString();
           }

           #endregion
       }

       private static class Win32
       {

       [DllImport("user32.dll")]
       public static extern IntPtr GetForegroundWindow();

       [DllImport("user32.dll", EntryPoint = "GetWindowTextA")]
       public static extern int GetWindowText(
            IntPtr hwnd,
            StringBuilder lpString,
            int cch);

       [DllImport("kernel32.dll", EntryPoint = "FormatMessageA")]
       public static extern int FormatMessage(
            int dwFlags,
            int lpSource,
            int dwMessageId,
            int dwLanguageId,
            StringBuilder lpBuffer,
            int nSize,
            int Arguments);

       }

   }
}