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to send a message to another pc, first you must be using professional version or ultimate version.
assuming that you do,
before you do this, back up your system. messing with the registry files is a risky business

First, in the search box in the start menu, type in regedit. double click hkey_local machine, then system, then CurrentControlSet, then Control, then terminal server, and finally in the window on the right, double click AllowRemoteRPC. Change the value data to 1 and then click ok

 Second, in the start menu search box, type credential manager. In credential Manager click on add a Windows Credential. The top box wants the name of the remote computer. the second box wants the username that you will be messaging. the third box wants the password of the username of the remote computer.

Third, restart your computer

Fourth, you should be ready to send a message. Open a command prompt window. type msg /server nameofremotepchere usernamehere messagehere

good luck

ps the error 5 is because of the hkey problem, in case you want to know