On Windows 7 there is an option that displays a text message to the user of the computer on system start-up. This message is configurable in the Local Security Policy which you can access over the start menu's program search bar. Once in there, expand Local Policies on the left frame of the window and select Security Options. There, find the two entries named message title for users attempting to log on and message text for users attempting to log on.
As the names say, the value of the title message is later displayed as the title, same goes for the text. The text can hold an infinite amount of lines and approximately 94 characters in one line. Close the window after you have specified your desired entries.
Both of these settings are also available in the Windows Registry. To find them, open the registry by typing regedit into the Run... prompt and browse the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Policies\System key. There, the legalnoticecaption and the legalnoticetext values are stored. These are the registry storages for the values you entered in the Local Security Policy window.