I'm going to share you about a hidden option in windows 7, which helps you to add a message before login screen comes.
For this first open Run ( Windowskey + R ),
cmd /c reg add HKLM\software\microsoft\windows\currentversion\policies\system /v Legalnoticecaption /t REG_SZ /d aaaaaaa /f
cmd /c reg add HKLM\software\microsoft\windows\currentversion\policies\system /v Legalnoticetext /t REG_SZ /d aaaaaaa /f
Replace aaaaa with your desired messsage , type them on Run terminal and press enter.
