Windows 7, like Vista, supports a local security policy that allows you to create a message that will be displayed when users attempt to log on.
To create a logon message:
- Type secpol.msc in Start Search 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.
Incidentally, our Windows Logon Notfier makes the job very easy for you!
Also see Display custom legal notices & start up messages in Windows 7 | 8.