Yes, it can be done through the registry but it's also a Local Security Policy.
Are you running NT, 2000 or XP? Do you have Admin rights?
Just goto Control Panel, Administrative Tools > LSP > Local Policy > Security Options > Interactive Logon: Message text for users attempting to log on
And then the one below it "Message title....."
For 2000, it's basically in the same place, just has a little different verbiage.