Windows 8, like Windows 7 and Windows Vista, supports a local security policy, that allows you to create a message that will be displayed when users attempt to log on to a Windows computer. This can be implemented with the help of Group Policy.
Create a logon message
To create a logon message:
Run secpol.msc and hit Enter to open the Local Security Policy
Expand Local Policies > Select Security options.
In the RHS pane, double click Interactive logon: Message text for users attempting to log on.
In the provided box, type the message you want to appear.
This security setting specifies a text message that is displayed to users when they log on. This text is often used for legal reasons, for example, to warn users about the ramifications of misusing company information or to warn them that their actions may be audited. The default is, No message.
Click Apply/OK and Exit.
Incidentally, our Windows Logon Notfier makes the job very easy for you!
Also see how to display custom legal notices in Windows, using Windows Registry.