If you want to get some message on startup window when you log on your computer please follow these steps.
RUN-> REGEDIT-> HKEY_LOCAL_MACHINE-> SOFTWARE-> MICROSOFT-> WINDOWS NT-> CURRENT VERSION-> WINLOGIN
Now when you click on winlogin in the right hand side panel you see the values "Legalnoticecaption" and "Legalnoticetext". Double click on these values and write your own text which you want to see in your message on startup.