First, before editing your registry remember to always back it up first, just incase you do something wrong. The registry when modified is an instant change and cannot be undone.
BACK UP REGISTRY: So to open the registry editor click on "START", "RUN", then type "REGEDIT" . The registry editor will now open, click on "FILE" then "EXPORT". From the save window that appear type in a file name (ie regback.reg), now click on "SAVE". Your registry has now been backed up.
TO ADD ENTRY DO THE FOLLOWING: Click on the left hand panel click on "HKEY_LOCAL_MACHINE" , "Software", "Microsoft", "Windows NT", "CurrentVersion" then "WinLogon".
On the right hand panel find "LegalNoticeCaption", double click on it and add the title text that you wish to use, i.e. "WARNING" or "HELLO" or what ever you want. Click on OK to confirm.
Next double click on "LegalNoticeText" and add your message, i.e. "Don't delete files from my computer" or "Welcome to my computer" or what ever you want. Click on OK to confirm.
When you reboot a message screen will appear before the login screen with your message. You can put quite allot of text in the message.
