Computer Configuration/Policies/Windows Settings/Security Settings/Local Policies/Security Options: Interactive logon: Message text for users attempting to log on
Search Administrative Tools or for older version of Windows you can find it from the classic view in the control panel.
This I know is a feature in Windows for versions that support local GPO, you can configure a message to be displayed when a user attempts to logon. Usually when pressing Ctr+Alt+Del.
It basically displays a splash Window that you can customize to add legal information or warnings. (see screenshot at the bottom of the answer)
Being defined by group policy you can of course use this in a Windows server environment for multiple/specific machines.
I remember stumbling on the setting years ago and I believe users are forced to press "OK" to the message window before being able to logon.
This of course is a Windows solution and a good one in my opinion, however I would assume third party software exists for versions when the user is logged on. I would have to Google that which I'm sure you have done.
In my opinion the agreement before logon is stronger than once the user is using the machine. This is one of the features that sprung instantly to mind from reading your question so I hope it's what you're looking for.
Image of the screen: