Thats pretty simple.
If you go to the registry editor, you can edit and change the wording of the message as you like.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
I attached a .txt file wich will make it even easier.
Just rename the file to a .reg extension when you are done changing the message and save it.
All you have to do is double-click the file and it edits the registry automatically for you. You can edit it by right clicking > edit then everything below the [registry entry] can be changed with a message.