If the computer is networked / part of a domain / work pc you probably won't have any luck removing the legal notice.
If it is a standalone pc you remove remove the notification with a registry edit.
Whenever making changes to the registry it is very important that it is backed up first. If you are in any way unsure seek help from someone familiar with registry editing.
Incorrect registry changes can render the operating system useless.
Click Start > Run and type in (or copy / paste):
Regedit
when the registry editor has opened hit the F3 key and type in (or copy / paste):
legalnoticecaption
hit enter.
When the search finds a match, in the right pane you will notice highlighted "legalnoticecaption" and right underneat it you will see "legalnoticetext".
The caption is the header of the notification and the text is the body.
To edit out the warning double click legalnoticecaption and delete the text from Value Data and click ok. Do the same for legalnoticetext.
Now when you restart your pc the notification shouldn't appear before the login / password prompt.