This is a reminder on how can we send messages to Windows7/2008 after Microsoft deprecated net send mechanism.
Here I want to send message from my Windows XP SP3 workstation to Windows 7 SP1 machine. I’ll be using with msg.exe, the replacement for net send.

E:\>msg /?
Send a message to a user.

MSG {username | sessionname | sessionid | @filename | *}
    [/SERVER:servername] [/TIME:seconds] [/V] [/W] [message]

  username            Identifies the specified username.
  sessionname         The name of the session.
  sessionid           The ID of the session.
  @filename           Identifies a file containing a list of usernames,
                      sessionnames, and sessionids to send the message to.
  *                   Send message to all sessions on specified server.
  /SERVER:servername  server to contact (default is current).
  /TIME:seconds       Time delay to wait for receiver to acknowledge msg.
  /V                  Display information about actions being performed.
  /W                  Wait for response from user, useful with /V.
  message             Message to send.  If none specified, prompts for it
                      or reads from stdin.

My first attempt failed with error:

This was executed on my XP workstation trying 
to send message to "me" logged on mywindows7 PC:
E:\>msg alesk /server:mywindows7  "I need some coffee!!"
Error opening Terminal server kavsek7
Error [1717]:The interface is unknown.

The solution is to allow remote RPC execution on MYWINDOWS7 machine. On MYWINDOWS7 I opened Registry editor and changed AllowRemoteRPC from 0 to 1:

HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server
Name : AllowRemoteRPC
Type : REG_DWORD
Value : 1

Of course, it’s completely up to you to decide if you can live with open RPC port.