Introduction:
Message queuing is often used as a backup system for whenever a communication link fails between two servers. This improves overall system stability in the event of catastrophic failure. This type of fallback mechanism is vital in systems where out-of-sync data between sites could cause opportunities for malicious users to defraud the system

Main:
MSMQ is included with windows XP,for installing MSMQ jut go to
Start–>Control Panel–>Administrative Tools–>Computer Management–>Services and Applications–>Message Queuing

To Implementing MSMQ in .NET Programming just add System.Messaging.dll
See this sample Code,

private void btnSend_Click(object sender, System.EventArgs e)
{
  string queueName = “.\\private$\\sample”;
  MessageQueue mq;
  if (MessageQueue.Exists(queueName))
  {
    mq=new MessageQueue(queueName);
  }
  else
  {
    mq = MessageQueue.Create(queueName);
  }
  mq.Send(tbMessage.Text);
}

This code first looks at MSMQ to see if a queue of the name \private$\sample has been created on the local machine.
If it has not, then a MessageQueue object points to the existing one. The contents of the textbox (tbMessage) are then sent as a message to this queue.

Common MSMQ Classes are,

Formatter
 Specifies the formatter used to serialize or deserialize the message body; can be either XmlMessageFormatter, ActiveXMessageFormatter, or BinaryMessageFormatter
 
Label
 Specifies a human-readable queue description
 
Path
 Specifies the location of the queue
 
Transactional
 Specifies whether the queue can accept nontransactional messages
 
Authenticate
 Specifies whether the queue can accept unauthenticated messages
 
EncryptionRequired
 Specifies whether the queue can accept unencrypted messages
 
Close
 Frees all resources used by the handle to the queue
 
Create
 Creates a new queue at the specified path
 
Delete
 Removes a queue from MSMQ, deleting all messages contained therein
 
GetAllMessages
 Returns an array of messages from the specified queue
 
GetPrivateQueuesByMachine
 Returns an array of private message queues from the specified machine
 
GetPublicQueues
 Returns an array of queues on the local network
 
Receive
 Returns a message from the top of the specified queue
 
Send
 Sends a message to the tail of the specified queue
 
Purge
 Deletes all messages from a queue, but does not delete the queue itself
 
Conclusion:
MSMQ is the key Programming essential for disconnected architecture,
Hope this helps,